cc: Remove ReadPixels from compositor layer_unittests.cc

BUG=332167

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245601

Review URL: https://codereview.chromium.org/133273026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248068 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248068