Bug 1383982 - Introduce a general mechanism for measuring memory usage of graph-like structures. r=erahm.

All the SizeOf{In,Ex}cludingThis() functions take a MallocSizeOf function
which measures memory blocks. This patch introduces a new type, SizeOfState,
which includes a MallocSizeOf function *and* a table of already-measured
pointers, called SeenPtrs. This gives us a general mechanism to measure
graph-like data structures, by recording which nodes have already been
measured. (This approach is used in a number of existing reporters, but not in
a uniform fashion.)

The patch also converts the window memory reporting to use SizeOfState in a lot
of places, all the way through to the measurement of Elements. This is a
precursor for bug 1383977 which will measure Stylo elements, which involve
Arcs.

The patch also converts the existing mAlreadyMeasuredOrphanTrees table in the
OrphanReporter to use the new mechanism.

dom/html/HTMLLinkElement.cpp

@@ -509,10 +509,10 @@ HTMLLinkElement::IntrinsicState() const
 }
 
 size_t
-HTMLLinkElement::SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+HTMLLinkElement::SizeOfExcludingThis(mozilla::SizeOfState& aState) const
 {
-  return nsGenericHTMLElement::SizeOfExcludingThis(aMallocSizeOf) +
-         Link::SizeOfExcludingThis(aMallocSizeOf);
+  return nsGenericHTMLElement::SizeOfExcludingThis(aState) +
+         Link::SizeOfExcludingThis(aState);
 }
 
 JSObject*