Make Android/iOS local/remote description accessors thread safe.

Since the descriptions can be modified on the signaling thread,
ToString can only be safely called on that thread.

Bug: webrtc:11791
Change-Id: Icf6aada8aa66d00be94c6bda7b22e41b5d3bbc17
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/180541
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Taylor <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31862}

api/peer_connection_proxy.h

@@ -147,6 +147,7 @@ PROXY_METHOD2(bool,
 PROXY_METHOD1(bool, StartRtcEventLog, std::unique_ptr<RtcEventLogOutput>)
 PROXY_METHOD0(void, StopRtcEventLog)
 PROXY_METHOD0(void, Close)
+BYPASS_PROXY_CONSTMETHOD0(rtc::Thread*, signaling_thread)
 END_PROXY_MAP()
 
 }  // namespace webrtc