[mux] Post bug fix and cleanup after r646
[mux] Post bug fix and cleanup after r646

- _LogicalStream._send_closing_handshake must create outer frame.
- Rewrite _LogicalStream.receive_message(). Just call
  Stream.receive_message() and catch a mux specific exception.
- Fix comment.
- Simplify error check.

BUG=none
TEST=test/run_all.py

Review URL: https://codereview.appspot.com/6346074

git-svn-id: d23f85197b9dafcb1b1682eb9d0eda70517bdc52@647 4ff78f4a-9131-11de-b045-6380ec9940d4

bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src