Ensure that ImageBitmaps that specify a source rect do not bleed color.

BUG=260958,166658

Review URL: https://chromiumcodereview.appspot.com/22438006

git-svn-id: svn://svn.chromium.org/blink/trunk@155660 bbb929c8-8fbe-4397-9dbb-9b2b20218538