Fix a race between Channel::Shutdown() and Channel::DetachEndpoint().

Fix a race between Channel::Shutdown() and Channel::DetachEndpoint().

This is a roll of
https://github.com/domokit/mojo/commit/457228e6bee9614d80ee04a98b296f96090304de

Original commit message:
The race is between shutdown and message pipe closing. Closing a message pipe
results in Channel::DetachEndpoint being called. The racing sequence is:
 IO Thread                            Other Thread
                               Channel::DetachEndpoint (return from
                               DetachEndpointInternal)
                               Channel::local_id_to_endpoint_map_ no
                               longer contains the message pipe's
                               endpoint, preventing Channel::Shutdown from
                               blocking on ChannelEndpoint::mutex_ in
                               ChannelEndpoint::DetachFromChannel().
Channel::Shutdown()
delete Channel
                               Channel::SendControlMessage
                               use-after-free

Review URL: https://codereview.chromium.org/1342073002

Cr-Commit-Position: refs/heads/master@{#349040}