Previously, GetRemoteAssociatedInterface() could drop the associated request if
the channel was not connected. It would lead to crash when the corresponding
associated interface pointer is used to make calls.

This CL makes the request associated with a dedicated, disconnected message pipe
instead of dropping it. That way it is safe to make calls with the interface
pointer.

BUG=704504

Review-Url: https://codereview.chromium.org/2772983002
Cr-Commit-Position: refs/heads/master@{#459642}