Unreviewed gardening. More flaky compositing tests.

TBR=rbyers@chromium.org
BUG=339118

Review URL: https://codereview.chromium.org/132123006

git-svn-id: svn://svn.chromium.org/blink/trunk@166043 bbb929c8-8fbe-4397-9dbb-9b2b20218538