Update MockMessageHandler to inherit from MessageHandlerAutoCleanup.

This is to facilitate interface changes in webrtc.
No functional change.

Bug: webrtc:11908
Change-Id: I19f56192ddddc93c0577cb76d163bfbb0ce6175b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406216
Auto-Submit: Tommi <tommi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806248}

jingle/glue/thread_wrapper_unittest.cc

@@ -32,7 +32,7 @@ static const int kMaxTestDelay = 40;
 
 namespace {
 
-class MockMessageHandler : public rtc::MessageHandler {
+class MockMessageHandler : public rtc::MessageHandlerAutoCleanup {
  public:
   MOCK_METHOD1(OnMessage, void(rtc::Message* msg));
 };