gtk: Only accept text/uri-list drops in the tab strip.  We were accepting text/plain before, which was giving us the html for a dragged image instead of the url of the image.
gtk: Only accept text/uri-list drops in the tab strip.  We were accepting text/plain before, which was giving us the html for a dragged image instead of the url of the image.

BUG=16165
TEST=Go to google.com.  Drag the Google logo into the tabstrip.  The image should open up in a new tab (or the current tab, depending on the drop).
Review URL: http://codereview.chromium.org/159212

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@21304 0039d316-1c4b-4281-b951-d872f2087c98

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