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_interface.h

@@ -909,6 +909,10 @@ class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface {
       const std::string& label,
       const DataChannelInit* config) = 0;
 
+  // NOTE: For the following 6 methods, it's only safe to dereference the
+  // SessionDescriptionInterface on signaling_thread() (for example, calling
+  // ToString).
+
   // Returns the more recently applied description; "pending" if it exists, and
   // otherwise "current". See below.
   virtual const SessionDescriptionInterface* local_description() const = 0;
@@ -1136,6 +1140,14 @@ class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface {
   // thus the observer object can be safely destroyed.
   virtual void Close() = 0;
 
+  // The thread on which all PeerConnectionObserver callbacks will be invoked,
+  // as well as callbacks for other classes such as DataChannelObserver.
+  //
+  // Also the only thread on which it's safe to use SessionDescriptionInterface
+  // pointers.
+  // TODO(deadbeef): Make pure virtual when all subclasses implement it.
+  virtual rtc::Thread* signaling_thread() const { return nullptr; }
+
  protected:
   // Dtor protected as objects shouldn't be deleted via this interface.
   ~PeerConnectionInterface() override = default;

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

api/proxy.h

@@ -400,11 +400,13 @@ class ConstMethodCall : public rtc::Message, public rtc::MessageHandler {
 // For use when returning purely const state (set during construction).
 // Use with caution. This method should only be used when the return value will
 // always be the same.
-#define BYPASS_PROXY_CONSTMETHOD0(r, method)                            \
-  r method() const override {                                           \
-    static_assert(!std::is_pointer<r>::value, "Type is a pointer");     \
-    static_assert(!std::is_reference<r>::value, "Type is a reference"); \
-    return c_->method();                                                \
+#define BYPASS_PROXY_CONSTMETHOD0(r, method)                                \
+  r method() const override {                                               \
+    static_assert(                                                          \
+        std::is_same<r, rtc::Thread*>::value || !std::is_pointer<r>::value, \
+        "Type is a pointer");                                               \
+    static_assert(!std::is_reference<r>::value, "Type is a reference");     \
+    return c_->method();                                                    \
   }
 
 }  // namespace webrtc

api/test/dummy_peer_connection.h

@@ -237,7 +237,11 @@ class DummyPeerConnection : public PeerConnectionInterface {
 
   void StopRtcEventLog() { FATAL() << "Not implemented"; }
 
-  void Close() {}
+  void Close() override {}
+
+  rtc::Thread* signaling_thread() const override {
+    return rtc::Thread::Current();
+  }
 };
 
 static_assert(