Fix tab capture with Mac software compositor

The tab capture API is waiting for the DID_UPDATE_BACKING_STORE
message, which is called by RenderWidgetHostImpl when the
backing store is updated, which is never for the software compositor.

This is not the most accurate use of the message, but the fix needs
to be backported to M34 and any renaming or repurposing of the
notification would be inadvisable.

BUG=333986

Review URL: https://codereview.chromium.org/178463013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254734 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254734
Revert of Move WebAXObjectProxy and AccessibleController from CppBoundClass to gin::Wrappable (https://codereview.chromium.org/172263002/)

Reason for revert:
fast/dom/Window/property-access-on-cached*.html tests (4 in total) assert on debug bots.

Original issue's description:
> Move WebAXObjectProxy and AccessibleController from CppBoundClass to gin::Wrappable
> 
> BUG=297480, 331301
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=253821
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=254432

TBR=jochen@chromium.org,kouhei@chromium.org,dmazzoni@chromium.org,haraken@chromium.org,hajimehoshi@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=297480, 331301

Review URL: https://codereview.chromium.org/186523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254735 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254735