Propagate window focus changes to plugins on the Mac

      This gives plugins mostly correct window activation information; there are still a few loose ends like initial focus state and changes that happen while plugins are in background tabs, but those will be handled in follow-up bugs.

BUG=31847
TEST=Plugins that are in background windows should act accordingly (e.g., Silverlight hover effects shouldn't trigger)

Review URL: http://codereview.chromium.org/549194

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@37694 0039d316-1c4b-4281-b951-d872f2087c98