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The echo message is essentially an async fence with event based notification. The client gets a notification when the GPU process has completed all the work up to the last flush.

I used it to replace the SwapBuffers / OnSwapBuffers synchronization and got rid of some of the callbacks in the lower layers of the stack.

The SwapBuffers callbacks in the GPU process are only needed on mac now and I will replace them with something more generic soon.
Review URL: http://codereview.chromium.org/7762013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@98747 0039d316-1c4b-4281-b951-d872f2087c98