[ash] Don't remove an item when associated window is dragged

LauncherItem adding/removing is repeated when a Window is dragged because
it is reparented to DockedContainer during the dragging.
ShelfWindowWatcher should prevent this removing when window is dragging.

R=sky@chromium.org
BUG=NONE
TEST=ash_unittests --gtest_filter=ShelfWindowWatcherTest*.*

Review URL: https://codereview.chromium.org/108313006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243970 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243970