Add GPU support for axis-aligned ovals:

Add GPU support for axis-aligned ovals:
- Add drawOval base function to SkDevice, and override in SkGpuDevice
- Move isSimilarityMatrix to SkMatrix (renamed to isSimilarity) and fixed up unit test
- Since both SkGpuDevice::drawOval() and GrContext::drawPath() can try to draw ovals, added GrContext::canDrawOval() and GrContext::internalDrawOval() to avoid duplicate code
- Hooked in axis-aligned oval fill shader
- Enabled GPU stroked circles
- Added stroked circle bench test

Review URL: https://codereview.appspot.com/7137050

git-svn-id: http://skia.googlecode.com/svn/trunk@7304 2bbb7eff-a529-9590-31e7-b0007b416f81