Enables delegating signal strategy for It2Me Host.

Motivation:
Allow the It2me host to delegates signaling messages sending/receiving
via native messaging.

Summary of changes:
- Add a new flag useSignalingProxy on the connect message for
  com.google.chrome.remote_assistance.
- Move delegating_signal_strategy from the plugin directory to the
  signaling directory so that it can be shared by the client and the
  host
- Change the threading behavior of delegating_signal_strategy so that
  the delegate and the listener can be run on two different threads.

Review-Url: https://codereview.chromium.org/2384063008
Cr-Commit-Position: refs/heads/master@{#423692}
17 files changed