Try to fix crash in WebSocketExperimentRunner::DoLoop

      Crash in WebSocketExperimentRunner::DoLoop() by access violation write.
I guess WebSocketExperimentRunner is deleted before calling back DoLoop.
I suspect WebSocketExperimentRunner is deleted if some task was finished
with ERR_ABORTED other than Cancel.  ERR_ABORTED may happen when SocketStream
closed unexpectedly, so it would not be good to handle Cancel case.
Use next_state_ == STATE_NONE to check the task was cancelled.

BUG=53985
TEST=none

Review URL: http://codereview.chromium.org/3317001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@58317 0039d316-1c4b-4281-b951-d872f2087c98