Add an image to mask the download shelf close button so that themes don't hide it.
Add an image to mask the download shelf close button so that themes don't hide
it.

BUG= http://crbug.com/18451
TEST= none


git-svn-id: http://src.chromium.org/svn/trunk/src@22970 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 3ff5f1e351e47c70b378ee76686426e04dd2538e
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser