Merge 135809 - [Chromium] Shared graphics context should only pushGroupMarker() once
Reviewed by James Robinson.
GraphicsContext3D's pushGroupMarkerEXT() is being called every time
an ImageBuffer is created, leading to unlimited memory growth, since
they share a common GraphicsContext3D. It should be called only once,
on context creation.
See also http://crbug.com/160411
Tested by manually checking the memory usage in Chrome's TaskManager.
(Sorry, I can't think of a way to test this automatically.)