Linux: Load PRIMARY selection on middle-click on new tab button.
Linux: Load PRIMARY selection on middle-click on new tab button.

BUG=18938,11612,11614
TESTED=highlighted text and checked that middle-click loads it if it's a URL but does nothing if it isn't

Review URL: http://codereview.chromium.org/164282

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@23025 0039d316-1c4b-4281-b951-d872f2087c98

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