Fixed race condition on disconnect for WebRTC browser tests.

We were hitting this error on some bots:
"Uncaught TypeError: Cannot call method 'addIceCandidate' of null",
source: http://127.0.0.1:50344/webrtc/jsep01_call.js (125)

We're not particularly worried about the shutdown sequence of events,
so I'll just discard message that arrive after disconnecting.

R=perkj@chromium.org
TBR=perkj@chromium.org
BUG=334192
NOTRY=True

Review URL: https://codereview.chromium.org/138043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244851 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244851