Remove the acceleratedCompositingForPluginsEnabled Setting.

It's no longer set to be anything but true.

Depends on: https://codereview.chromium.org/239973005/

R=abarth@chromium.org, vollick@chromium.org
BUG=365853

Review URL: https://codereview.chromium.org/247673005

git-svn-id: svn://svn.chromium.org/blink/trunk@173002 bbb929c8-8fbe-4397-9dbb-9b2b20218538