Bluetooth: always release the pairing context on Cancel

BUG=266078
TEST=cancelling Sony BT-00 does not crash Chrome

Review URL: https://codereview.chromium.org/172903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252468 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252468