NTP: Make sure that we do not focus the thumbnails on mouse down.
NTP: Make sure that we do not focus the thumbnails on mouse down.

We used to focus the thumbnail on mousedown which makes the thumbmail hover
effect show up which confuses users.

BUG=31361
TEST=Right click on a thumbnail and dismiss the context menu. The thumbnail
should not be focused.

Review URL: http://codereview.chromium.org/525045

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@35589 0039d316-1c4b-4281-b951-d872f2087c98

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