Rebaseline two compositing tests

These image results changed slightly after
http://src.chromium.org/viewvc/chrome?revision=250739&view=revision

TBR=danakj@chromium.org
BUG=342848

Review URL: https://codereview.chromium.org/160793003

git-svn-id: svn://svn.chromium.org/blink/trunk@167030 bbb929c8-8fbe-4397-9dbb-9b2b20218538