Make MessageHandler be a pure virtual interface.

Bug: webrtc:11908
Change-Id: I35d3c4005d970082bff8c5ff24186aab54205c37
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185340
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32194}
diff --git a/rtc_base/thread.h b/rtc_base/thread.h
index cd31da4..24ea356 100644
--- a/rtc_base/thread.h
+++ b/rtc_base/thread.h
@@ -534,7 +534,7 @@
  private:
   class QueuedTaskHandler final : public MessageHandler {
    public:
-    QueuedTaskHandler() : MessageHandler(false) {}
+    QueuedTaskHandler() {}
     void OnMessage(Message* msg) override;
   };