[Remoting Mobile] Centralized OAuth Token Getter

This CL:
* Provides an interface in ChromotingClientRuntime to allow mobile client
  to fetch OAuth token from a centralized place.
* Provides iOS implementation for the OAuthTokenGetter.
* Passes the token getter to the transport context so that it can be used
  to fetch the ice config.

The centralized OAuthTokenGetter will be used for WebRTC protocol and
will also be used for telemetry logging. I'll send a follow up CL to
implement the OAuthTokenGetter for Android and refactor the auth logic
in telemetry logger.

Bug: 780736
Change-Id: I91b6f3dac8271e27b18854daddd52637a0a66add
Reviewed-on: https://chromium-review.googlesource.com/754410
Reviewed-by: Jamie Walch <[email protected]>
Commit-Queue: Yuwei Huang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#515020}
diff --git a/remoting/client/chromoting_client_runtime.cc b/remoting/client/chromoting_client_runtime.cc
index 28221c2..d9933a0 100644
--- a/remoting/client/chromoting_client_runtime.cc
+++ b/remoting/client/chromoting_client_runtime.cc
@@ -114,5 +114,8 @@
   return log_writer_.get();
 }
 
+OAuthTokenGetter* ChromotingClientRuntime::token_getter() {
+  return delegate_->token_getter();
+}
 
 }  // namespace remoting