Add support for Attachment Brokering of IPC::Channels on multiple threads.

Attachment brokering makes the assumption that there's a single thread
on which IO related to IPC::Channels occurs. This assumption is violated
when the flag --single-process is set.

BUG=590213

Review URL: https://codereview.chromium.org/1739203004

Cr-Commit-Position: refs/heads/master@{#382431}
12 files changed