Merge 120490 - Provide WillInitiatePaint() as well as DidInitiatePaint() and tweak semantics.

WillInitiatePaint() informs derived classes when the RenderWidget is about to perform a paint operation.

DidInitiatePaint() informs derived classes that the RenderWidget has finished the paint operation and sent it to the browser to display.

DidFlushPaint() informs derived classes when the paint operation has actually been displayed (either in response to UpdateRectAck, or OnSwapBuffersComplete()).

Pepper Graphics2D uses WillInitiatePaint and DidFlushPaint to trigger its Flush callback only for flushes occurring after the paint has actually been initiated.

Pepper Graphics3D uses DidInitiatePaint to know when the 3D pipeline is done with buffers from the previous paint operation, so that new content rendered to them won't be prematurely flushed to the screen.

Known issues with this CL:

* There is still scope for the Graphics2D to get confused when threaded compositing is in use, since DidFlushPaint for an older paint may be received after WillInitiatePaint for a more recent one, but that is a fix for another CL.

* Graphics3D will now be notified earlier in than it would have been in the software case, since it previously hooked DidFlushPaint.

BUG=111639
TEST=Run Native Client Examples "Pi generator", "Tumbler" and "Fullscreen Tumbler"  with and without --force-composite-mode and verify they update correctly.


Review URL: http://codereview.chromium.org/9328016

TBR=wez@chromium.org
Review URL: https://chromiumcodereview.appspot.com/9360027

git-svn-id: svn://svn.chromium.org/chrome/branches/1025/src@121038 0039d316-1c4b-4281-b951-d872f2087c98

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