Added GPU process "echo" IPC message.

Added GPU process "echo" IPC message.

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.
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=98747

[apatrick_chromium, 2011-08-27 01:04:07]

I'm not finished testing this but it's never too early to call me a lunatic.

[apatrick_chromium, 2011-08-29 21:05:39]

PTAL. Mac seems to work with patch set #2.

[Ken Russell (switch to Gerrit), 2011-08-29 21:50:41]

This LGTM. I don't have any substantial comments. The simplification looks good
and the memory management of the IPC messages looks correct.

[piman, 2011-08-29 22:06:31]

LGTM too.