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This just intersects the paint rect with the plugin rect (previously we used
the entire plugin rect, even when the paint rect was smaller).

TEST=manual
BUG=none
Review URL: http://codereview.chromium.org/6296010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72191 0039d316-1c4b-4281-b951-d872f2087c98