Bluetooth: avoid sending cancel after accept or reject

After clicking the Accept or Reject buttons, the pairing dialog is closed
which results in a CancelPairing method call being sent after the reply
to the original method call.

This isn't harmful right now, but could be a bug if BlueZ changes, so fix
it now.

BUG=none
TEST=verify with d-bus monitor that only a method reply is sent

Review URL: https://codereview.chromium.org/148643003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249832 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249832