[message_loop] Clean up MessageLoopCurrent a bit.

- Do not allow assigning MessageLoop to MessageLoopCurrent.
- Remove hash operator from MessageLoopCurrent.

R=alexclarke@chromium.org,gab@chromium.org
TBR=gab@chromium.org
BUG=891670

Change-Id: If558357328d1b07abaa5ccf7bcba5f9ef3609531
Reviewed-on: https://chromium-review.googlesource.com/c/1322874Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606033}

base/message_loop/message_loop_current.cc

@@ -40,11 +40,6 @@ bool MessageLoopCurrent::IsSet() {
   return !!GetTLSMessageLoop()->Get();
 }
 
-MessageLoopCurrent& MessageLoopCurrent::operator=(MessageLoop* message_loop) {
-  current_ = message_loop;
-  return *this;
-}
-
 void MessageLoopCurrent::AddDestructionObserver(
     DestructionObserver* destruction_observer) {
   DCHECK_CALLED_ON_VALID_THREAD(current_->bound_thread_checker_);

base/message_loop/message_loop_current.h

@@ -58,10 +58,6 @@ class BASE_EXPORT MessageLoopCurrent {
   MessageLoopCurrent(MessageLoopCurrent&& other) = default;
   MessageLoopCurrent& operator=(const MessageLoopCurrent& other) = default;
 
-  // TODO(altimin): Remove this. Currently it's used in places where the caller
-  // has access to MessageLoop due to owning it.
-  MessageLoopCurrent& operator=(MessageLoop* message_loop);
-
   bool operator==(const MessageLoopCurrent& other) const;
 
   // Returns a proxy object to interact with the MessageLoop running the
@@ -209,7 +205,6 @@ class BASE_EXPORT MessageLoopCurrent {
   friend class Thread;
   friend class sequence_manager::LazyThreadControllerForTest;
   friend class sequence_manager::internal::SequenceManagerImpl;
-  friend struct std::hash<MessageLoopCurrent>;
   friend class MessageLoopTaskRunnerTest;
   friend class web::TestWebThreadBundle;
 
@@ -332,15 +327,4 @@ class BASE_EXPORT MessageLoopCurrentForIO : public MessageLoopCurrent {
 
 }  // namespace base
 
-namespace std {
-
-template <>
-struct hash<base::MessageLoopCurrent> {
-  size_t operator()(const base::MessageLoopCurrent& loop) {
-    return std::hash<void*>()(loop.current_);
-  }
-};
-
-}  // namespace std
-
 #endif  // BASE_MESSAGE_LOOP_MESSAGE_LOOP_CURRENT_H_