Bug 1387956 - Overhaul ComputedValues measurement, and add style structs measurement. r=bholley.

This patch moves measurement of ComputedValues objects from Rust to C++.
Measurement now happens (a) via DOM elements and (b) remaining elements via
the frame tree. Likewise for the style structs hanging off ComputedValues
objects.

Here is an example of the output.

> ├──27,600,448 B (26.49%) -- active/window(https://en.wikipedia.org/wiki/Barack_Obama)
> │  ├──12,772,544 B (12.26%) -- layout
> │  │  ├───4,483,744 B (04.30%) -- frames
> │  │  │   ├──1,653,552 B (01.59%) ── nsInlineFrame
> │  │  │   ├──1,415,760 B (01.36%) ── nsTextFrame
> │  │  │   ├────431,376 B (00.41%) ── nsBlockFrame
> │  │  │   ├────340,560 B (00.33%) ── nsHTMLScrollFrame
> │  │  │   ├────302,544 B (00.29%) ── nsContinuingTextFrame
> │  │  │   ├────156,408 B (00.15%) ── nsBulletFrame
> │  │  │   ├─────73,024 B (00.07%) ── nsPlaceholderFrame
> │  │  │   ├─────27,656 B (00.03%) ── sundries
> │  │  │   ├─────23,520 B (00.02%) ── nsTableCellFrame
> │  │  │   ├─────16,704 B (00.02%) ── nsImageFrame
> │  │  │   ├─────15,488 B (00.01%) ── nsTableRowFrame
> │  │  │   ├─────13,776 B (00.01%) ── nsTableColFrame
> │  │  │   └─────13,376 B (00.01%) ── nsTableFrame
> │  │  ├───3,412,192 B (03.28%) -- servo-style-structs
> │  │  │   ├──1,288,224 B (01.24%) ── Display
> │  │  │   ├────742,400 B (00.71%) ── Position
> │  │  │   ├────308,736 B (00.30%) ── Font
> │  │  │   ├────226,512 B (00.22%) ── Background
> │  │  │   ├────218,304 B (00.21%) ── TextReset
> │  │  │   ├────214,896 B (00.21%) ── Text
> │  │  │   ├────130,560 B (00.13%) ── Border
> │  │  │   ├─────81,408 B (00.08%) ── UIReset
> │  │  │   ├─────61,440 B (00.06%) ── Padding
> │  │  │   ├─────38,176 B (00.04%) ── UserInterface
> │  │  │   ├─────29,232 B (00.03%) ── Margin
> │  │  │   ├─────21,824 B (00.02%) ── sundries
> │  │  │   ├─────20,080 B (00.02%) ── Color
> │  │  │   ├─────20,080 B (00.02%) ── Column
> │  │  │   └─────10,320 B (00.01%) ── Effects
> │  │  ├───2,227,680 B (02.14%) -- computed-values
> │  │  │   ├──1,182,928 B (01.14%) ── non-dom
> │  │  │   └──1,044,752 B (01.00%) ── dom
> │  │  ├───1,500,016 B (01.44%) ── text-runs
> │  │  ├─────492,640 B (00.47%) ── line-boxes
> │  │  ├─────326,688 B (00.31%) ── frame-properties
> │  │  ├─────301,760 B (00.29%) ── pres-shell
> │  │  ├──────27,648 B (00.03%) ── pres-contexts
> │  │  └─────────176 B (00.00%) ── style-sets

The 'servo-style-structs' and 'computed-values' sub-trees are new. (Prior to
this patch, ComputedValues under DOM elements were tallied under the the
'dom/element-nodes' sub-tree, and ComputedValues not under DOM element were
ignored.) 'servo-style-structs/sundries' aggregates all the style structs that
are smaller than 8 KiB.

Other notable things done by the patch are as follows.

- It significantly changes the signatures of the methods measuring nsINode and
  its subclasses, in order to handle the tallying of style structs separately
  from element-nodes. Likewise for nsIFrame.

- It renames the 'layout/style-structs' sub-tree as
  'layout/gecko-style-structs', to clearly distinguish it from the new
  'layout/servo-style-structs' sub-tree.

- It adds some FFI functions to access various Rust-side data structures from
  C++ code.

- There is a nasty hack used twice to measure Arcs, by stepping backwards from
  an interior pointer to a base pointer. It works, but I want to replace it
  with something better eventually. The "XXX WARNING" comments have details.

- It makes DMD print a line to the console if it sees a pointer it doesn't
  recognise. This is useful for detecting when we are measuring an interior
  pointer instead of a base pointer, which is bad but easy to do when Arcs are
  involved.

- It removes the Rust code for measuring CVs, because it's now all done on the
  C++ side.

MozReview-Commit-ID: BKebACLKtCi

dom/html/HTMLAreaElement.cpp

@@ -219,11 +219,13 @@ HTMLAreaElement::IntrinsicState() const
   return Link::LinkState() | nsGenericHTMLElement::IntrinsicState();
 }
 
-size_t
-HTMLAreaElement::SizeOfExcludingThis(mozilla::SizeOfState& aState) const
+void
+HTMLAreaElement::AddSizeOfExcludingThis(SizeOfState& aState,
+                                        nsStyleSizes& aSizes,
+                                        size_t* aNodeSize) const
 {
-  return nsGenericHTMLElement::SizeOfExcludingThis(aState) +
-         Link::SizeOfExcludingThis(aState);
+  nsGenericHTMLElement::AddSizeOfExcludingThis(aState, aSizes, aNodeSize);
+  *aNodeSize += Link::SizeOfExcludingThis(aState);
 }
 
 JSObject*