added WritePixelsBench

This bench attempts to recreate Chromium's Canvas_putImageData performance test to track down https://code.google.com/p/chromium/issues/detail?id=270649.

Cranking the repitition count up to 500 on a MacPro yields:

WITHOUT 10624:
running bench [640 480]            writepix_rgba_UPM  8888: cmsecs = 748.25   GPU: cmsecs = 225.95 gmsecs = 244.80
running bench [640 480]             writepix_rgba_PM  8888: cmsecs =  54.12   GPU: cmsecs = 225.77 gmsecs = 244.59

WITH 10624:
running bench [640 480]            writepix_rgba_UPM  8888: cmsecs = 747.70   GPU: cmsecs = 230.30 gmsecs = 249.51
running bench [640 480]             writepix_rgba_PM  8888: cmsecs =  55.05   GPU: cmsecs = 230.13 gmsecs = 249.38

Committed: http://code.google.com/p/skia/source/detail?r=10761

[bsalomon, 2013-08-15 20:40:54]

lgtm

[commit-bot: I haz the power, 2013-08-16 00:30:08]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/robertphillips@google.com/22895012/3001

[commit-bot: I haz the power, 2013-08-16 00:40:39]

Change committed as 10761