linux_aura: Don't update cursor while shutting down the move loop.

During a drag and drop, we use a nested message loop and post a task to
signal termination of the drag. However, there might be more XdndStatus
messages in queue between the mouse release event and the posted
QuitClosure, so don't try to grab the cursor after we've already
released the cursor grab since that will persist past the end of the
move loop.

BUG=none
R=sadrul@chromium.org, varkha@chromium.org

Review URL: https://codereview.chromium.org/156183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249724 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249724