linux-aura: Restore middle-click on new-tab button behaviour.

Middle-clicking on new-tab button opens a new tab and navigates to the
URL in the clipboard, or navigates to the search-result page for the
clipboard content if it's not a URL.

BUG=319011
R=sky@chromium.org

Review URL: https://codereview.chromium.org/74133003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235469 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235469