Enable virtual/softwarecompositing tests on Mac.

We use softwarecompositing on mac now, so we really should
run the tests.

BUG=235110

Review URL: https://codereview.chromium.org/297833003

git-svn-id: svn://svn.chromium.org/blink/trunk@174751 bbb929c8-8fbe-4397-9dbb-9b2b20218538