Bug 1268852. Change <label> elements to not be form-associated anymore. r=bkelly,hsivonen,surkov

The web platform tests changes are just a cherrypick of
https://github.com/w3c/web-platform-tests/pull/2926 so I don't have to add
failure annotations until the next test uplift.

I've audited our uses of nsIFormControl, and this patch looks to me like it
preserves existing behavior in all but the following cases:

1)  nsXBLPrototypeHandler::DispatchXBLCommand, the case of scrolling when space
is pressed while something inside a <label> is focused.  We used to not scroll
in this situation; I think this is a bug, so I'm changing that behavior to
scroll instead.

2)  In Accessible::RelationByType for the RelationType::DEFAULT_BUTTON case,
when mContent is a <label> we used to return its form's default submit element.
Now we will just return Relation().

dom/html/HTMLFormControlsCollection.cpp

@@ -63,7 +63,9 @@ HTMLFormControlsCollection::ShouldBeInElements(nsIFormControl* aFormControl)
   // form.elements array
   //
   // NS_FORM_INPUT_IMAGE
-  // NS_FORM_LABEL
+  //
+  // XXXbz maybe we should just check for that type here instead of the big
+  // switch?
 
   return false;
 }