If we're in the middle of a drag, don't allow the user to middle click to close or right click for the context menu.
If we're in the middle of a drag, don't allow the user to middle
click to close or right click for the context menu.

BUG=21627
TEST=On a tab, left mouse down, right mouse click, then drag.  Left mouse
  up should release the tab.

Review URL: http://codereview.chromium.org/305004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@29542 0039d316-1c4b-4281-b951-d872f2087c98

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