Fix crash when download shelf had no theme.
Fix crash when download shelf had no theme.

BUG=http://crbug.com/36951
TEST=quit with download shelf open; don't crash

Review URL: http://codereview.chromium.org/660300

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@40293 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser