Pass --force-compositing-mode for virtual/stable/webexposed.
Pass --force-compositing-mode for virtual/stable/webexposed.

This is the last virtual suite without the flag. The tests all pass
on linux.

R=enne
BUG=361729

Review URL: https://codereview.chromium.org/284563003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@173903 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs