Mojo: Don't do any work in ProxyMessagePipeEndpoint's destructor.

In particular, detach from the ChannelEndpoint earlier.

(The ChannelEndpoint has a pointer to the MessagePipe that owns the
ProxyMessagePipeEndpoint. It should always be valid. Detaching in the
ProxyMessagePipeEndpoint's destructor means that that pointer is
momentarily not really valid -- "not really" in that the memory still
exists, but it's in the middle of the MessagePipe's destruction.)

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/619623003

Cr-Commit-Position: refs/heads/master@{#297673}