Fix bookmark dropping onto chrome://system page

Small change so we don't cancel drag and drop events unless the drop is a file.
Specifically, dragging a bookmark onto the page should cause the browser to
navigate to that URL.

BUG=336126
R=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/192373009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256175 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256175