Revert 245601 "cc: Remove ReadPixels from compositor layer_unitt..."

Broke compositor_unittests on Linux Aura Tests with error:

  LayerWithRealCompositorTest.DrawPixels (run #1):
  [ RUN ] LayerWithRealCompositorTest.DrawPixels
  ../../ui/compositor/layer_unittest.cc:832: Failure
  Value of: ReadPixels(&bitmap, gfx::Rect(viewport_size))
  Actual: false
  Expected: true
  [ FAILED ] LayerWithRealCompositorTest.DrawPixels (246 ms)

  LayerWithRealCompositorTest.DrawPixels (run #2):
  [ RUN ] LayerWithRealCompositorTest.DrawPixels
  ../../ui/compositor/layer_unittest.cc:832: Failure
  Value of: ReadPixels(&bitmap, gfx::Rect(viewport_size))
  Actual: false
  Expected: true
  [ FAILED ] LayerWithRealCompositorTest.DrawPixels (272 ms)

  LayerWithRealCompositorTest.DrawPixels (run #3):
  [ RUN ] LayerWithRealCompositorTest.DrawPixels
  ../../ui/compositor/layer_unittest.cc:832: Failure
  Value of: ReadPixels(&bitmap, gfx::Rect(viewport_size))
  Actual: false
  Expected: true
  [ FAILED ] LayerWithRealCompositorTest.DrawPixels (259 ms)

  LayerWithRealCompositorTest.DrawPixels (run #4):
  [ RUN ] LayerWithRealCompositorTest.DrawPixels
  ../../ui/compositor/layer_unittest.cc:832: Failure
  Value of: ReadPixels(&bitmap, gfx::Rect(viewport_size))
  Actual: false
  Expected: true
  [ FAILED ] LayerWithRealCompositorTest.DrawPixels (229 ms)

> cc: Remove ReadPixels from compositor layer_unittests.cc
> 
> BUG=332167
> 
> Review URL: https://codereview.chromium.org/133273026

TBR=enne@chromium.org

Review URL: https://codereview.chromium.org/131743027

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245673 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245673