Dont composite if rgba is not supported.
Dont composite if rgba is not supported.

TBR=derat@chromium.org
BUG=none
TEST=ScreenLockerTest.TestBasic passes.

Review URL: http://codereview.chromium.org/5121003

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@66443 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser