Make the pixel tests pass with software rasterization.
Make the pixel tests pass with software rasterization.

On the Aura display path, the pipe to screen is longer (GPU process passes texture for the browser to display). We need to ensure a certain number of frames are drawn before readback to guarantee that the pixels have hit the screen.

BUG=113624
TEST=go to chrome://tracing and see that HandleSwapBuffers is called 5 times in the GPU process for pixel_webgl.html

Review URL: http://codereview.chromium.org/9380011

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@121490 0039d316-1c4b-4281-b951-d872f2087c98

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