Do not consider drag capture window activatable for window selection.
This prevents the drag capture window from being included in overview mode.
BUG=325221
TEST=WindowSelectorTest.DragDropInProgress, RootWindowTest.CaptureWindow*
TEST=Select some text or an image and drag it to begin a drag and drop action. Then press F5. There should be no extra "blank" window in overview.
Review URL: https://codereview.chromium.org/
144363014
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249205 0039d316-1c4b-4281-b951-d872f2087c98