Most plugins want to be opaque, and notifying webkit of that will allow it to avoid rendering tiles behind the plugin and doing alpha blending.
BUG=116205
TEST=
Review URL: http://codereview.chromium.org/9580030
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@125100 0039d316-1c4b-4281-b951-d872f2087c98