Make MessageHandler cleanup optional.

As documented in webrtc:11908 this cleanup is fairly invasive and
when a part of a frequently executed code path, can be quite costly
in terms of performance overhead. This is currently the case with
synchronous calls between threads (Thread) as well with our proxy
api classes.

With this CL, all code in WebRTC should now either be using MessageHandlerAutoCleanup
or calling MessageHandler(false) explicitly.

Next steps will be to update external code to either depend on the
AutoCleanup variant, or call MessageHandler(false).

Changing the proxy classes to use TaskQueue set of concepts instead of
MessageHandler. This avoids the perf overhead related to the cleanup
above as well as incompatibility with the thread policy checks in
Thread that some current external users of the proxies would otherwise
run into (if we were to use Thread::Send() for synchronous call).

Following this we'll move the cleanup step into the AutoCleanup class
and an RTC_DCHECK that all calls to the MessageHandler are setting
the flag to false, before eventually removing the flag and make
MessageHandler pure virtual.

Bug: webrtc:11908
Change-Id: Idf4ff9bcc8438cb8c583777e282005e0bc511c8f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183442
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32049}

rtc_base/socket_unittest.cc

@@ -696,7 +696,7 @@ void SocketTest::DeleteInReadCallbackInternal(const IPAddress& loopback) {
   EXPECT_TRUE_WAIT(deleter.deleted(), kTimeout);
 }
 
-class Sleeper : public MessageHandler {
+class Sleeper : public MessageHandlerAutoCleanup {
  public:
   void OnMessage(Message* msg) override { Thread::Current()->SleepMs(500); }
 };