cc: Add some early outs to avoid expensive operations

cc: Add some early outs to avoid expensive operations

mapHomgenousPoint is not cheap. We can do faster in projectClippedRect similar
to mapClippedRect.

rectSubtractRegion constructs a Region to do subtraction from. If the
subtraction's rvalue is empty, then there is nothing to do and we can
avoid creating a Region structure at all.

I measured time spent in calculateRenderPasses with a draw-heavy unit test.

Baseline: 0.001148 seconds
With these changes: 0.001116 seconds

So, about 3% reduction in time spent in calculateRenderPasses.

R=enne
BUG=159718
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=166841

[enne (OOO), 2012-11-09 00:16:29]

3% is kind of in the noise, but these checks don't add much complexity.  lgtm.

[commit-bot: I haz the power, 2012-11-09 00:16:58]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/danakj@chromium.org/11312154/4

[commit-bot: I haz the power, 2012-11-09 02:02:20]

Change committed as 166841