Only composite opacity animations when already in compositing mode

This reverts a behavior change introduced by r161636. Entering compositing
mode for opacity animations is the suspected cause of a memory usage
regression on OS X 10.6.

BUG=317747

Review URL: https://codereview.chromium.org/63813004

git-svn-id: svn://svn.chromium.org/blink/trunk@162118 bbb929c8-8fbe-4397-9dbb-9b2b20218538