GTK: Drags of images should have titles instead of "".

BUG=32132
TEST=Drag an image from the content area to the bookmark bar. It should have a title.

Review URL: http://codereview.chromium.org/5307005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@67761 0039d316-1c4b-4281-b951-d872f2087c98