GpuScheduler using time quantum to avoid too many PostTasks

GpuScheduler fix: 
- Ganesh generates a lot of cheap GL commands, so 100 commands were executing in 50us, causing us to spin on PostTask(ProcessCommands) 60+ times per frame. This change simply tracks the elapsed time after each 100 commands - if the time is less than 2ms, it continues on to the next 100 commands. 

BUG=83628
TEST=trace FishIE demo with 500 fish, check that ProcessCommands is not called ~60 times per frame.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=87895

[jbates, 2011-06-03 21:28:48]

split #3

[apatrick_chromium, 2011-06-03 21:40:14]

LGTM

[piman, 2011-06-03 21:43:18]

LGTM