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Adds a couple of pixel-based compositor unittests. One to test that opacity is handled correctly, and one to check that MoveToFront and MoveAbove really do change the compositor output as expected.

Noteworthy changes are in layer_unittest.cc. Most of the changes below are due to moving compositor testing code into ui/gfx/compositor/test.

Depends on http://codereview.chromium.org/8463024/ and http://codereview.chromium.org/8590019/

BUG=None
TEST=compositor_unittests

Review URL: http://codereview.chromium.org/8533019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110657 0039d316-1c4b-4281-b951-d872f2087c98