Enable dragging of single tab between two windows.

To determine if a window is draggable, all that was needed is to call HasAvailableDrag action
instead of ContainsExactlyOneTab which was not we wanted anyways.

BUG=7861

Review URL: http://codereview.chromium.org/21525

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@10068 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed