Refactor GpuPixelBrowsertest.
Refactor GpuPixelBrowsertest.

Make it easier to add two more pixel tests, one for canvas 2d, and one for 3D CSs.

Note that this CL doesn't add or change any logic: it's a pure refactoring.

(This only affects GPU waterfall.)

BUG=
TEST=GPU waterfall: WebGLTeapot
R=jbates
Review URL: http://codereview.chromium.org/9202001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@117655 0039d316-1c4b-4281-b951-d872f2087c98

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