Remove unused external message loop support from base::Thread

The callsites were removed in crrev.com/618761.

Bug: 826465
Change-Id: If1b36e9b95b83183653bf5c927d8bdd8dbee7959
Reviewed-on: https://chromium-review.googlesource.com/c/1478893Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634300}

base/threading/thread.cc

@@ -208,15 +208,6 @@ void Thread::StopSoon() {
 
   stopping_ = true;
 
-  if (using_external_message_loop_) {
-    // Setting |stopping_| to true above should have been sufficient for this
-    // thread to be considered "stopped" per it having never set its |running_|
-    // bit by lack of its own ThreadMain.
-    DCHECK(!IsRunning());
-    message_loop_base_ = nullptr;
-    return;
-  }
-
   task_runner()->PostTask(
       FROM_HERE, base::BindOnce(&Thread::ThreadQuitHelper, Unretained(this)));
 }
@@ -272,19 +263,6 @@ bool Thread::GetThreadWasQuitProperly() {
   return quit_properly;
 }
 
-void Thread::SetMessageLoop(MessageLoop* message_loop) {
-  DCHECK(owning_sequence_checker_.CalledOnValidSequence());
-  DCHECK(message_loop);
-
-  // Setting |message_loop_base_| should suffice for this thread to be
-  // considered as "running", until Stop() is invoked.
-  DCHECK(!IsRunning());
-  message_loop_base_ = message_loop->GetMessageLoopBase();
-  DCHECK(IsRunning());
-
-  using_external_message_loop_ = true;
-}
-
 void Thread::ThreadMain() {
   // First, make GetThreadId() available to avoid deadlocks. It could be called
   // any place in the following thread initialization code.

base/threading/thread.h

@@ -248,16 +248,6 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   static void SetThreadWasQuitProperly(bool flag);
   static bool GetThreadWasQuitProperly();
 
-  // Bind this Thread to an existing MessageLoop instead of starting a new one.
-  // TODO(gab): Remove this after ios/ has undergone the same surgery as
-  // BrowserThreadImpl (ref.
-  // https://chromium-review.googlesource.com/c/chromium/src/+/969104).
-  void SetMessageLoop(MessageLoop* message_loop);
-
-  bool using_external_message_loop() const {
-    return using_external_message_loop_;
-  }
-
   // Returns the message loop for this thread.  Use the MessageLoop's
   // PostTask methods to execute code on the thread.  This only returns
   // non-null after a successful call to Start.  After Stop has been called,
@@ -331,13 +321,6 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   MessageLoopBase* message_loop_base_ = nullptr;
   RunLoop* run_loop_ = nullptr;
 
-  // True only if |message_loop_base_| was externally provided by |
-  // SetMessageLoop()| in which case this Thread has no underlying |thread_| and
-  // should merely drop |message_loop_base_| on Stop(). In that event, this
-  // remains true after Stop() was invoked so that subclasses can use this state
-  // to build their own cleanup logic as required.
-  bool using_external_message_loop_ = false;
-
   // Stores Options::timer_slack_ until the sequence manager has been bound to
   // a thread.
   TimerSlack timer_slack_ = TIMER_SLACK_NONE;

base/threading/thread_unittest.cc

@@ -522,61 +522,6 @@ TEST_F(ThreadTest, FlushForTesting) {
   a.FlushForTesting();
 }
 
-namespace {
-
-// A Thread which uses a MessageLoop on the stack. It won't start a real
-// underlying thread (instead its messages can be processed by a RunLoop on the
-// stack).
-class ExternalMessageLoopThread : public Thread {
- public:
-  ExternalMessageLoopThread() : Thread("ExternalMessageLoopThread") {}
-
-  ~ExternalMessageLoopThread() override { Stop(); }
-
-  void InstallMessageLoop() { SetMessageLoop(&external_message_loop_); }
-
-  void VerifyUsingExternalMessageLoop(
-      bool expected_using_external_message_loop) {
-    EXPECT_EQ(expected_using_external_message_loop,
-              using_external_message_loop());
-  }
-
- private:
-  base::MessageLoop external_message_loop_;
-
-  DISALLOW_COPY_AND_ASSIGN(ExternalMessageLoopThread);
-};
-
-}  // namespace
-
-TEST_F(ThreadTest, ExternalMessageLoop) {
-  ExternalMessageLoopThread a;
-  EXPECT_FALSE(a.task_runner());
-  EXPECT_FALSE(a.IsRunning());
-  a.VerifyUsingExternalMessageLoop(false);
-
-  a.InstallMessageLoop();
-  EXPECT_TRUE(a.task_runner());
-  EXPECT_TRUE(a.IsRunning());
-  a.VerifyUsingExternalMessageLoop(true);
-
-  bool ran = false;
-  a.task_runner()->PostTask(
-      FROM_HERE, base::BindOnce([](bool* toggled) { *toggled = true; }, &ran));
-  base::RunLoop().RunUntilIdle();
-  EXPECT_TRUE(ran);
-
-  a.Stop();
-  EXPECT_FALSE(a.task_runner());
-  EXPECT_FALSE(a.IsRunning());
-  a.VerifyUsingExternalMessageLoop(true);
-
-  // Confirm that running any remaining tasks posted from Stop() goes smoothly
-  // (e.g. https://codereview.chromium.org/2135413003/#ps300001 crashed if
-  // StopSoon() posted Thread::ThreadQuitHelper() while |run_loop_| was null).
-  base::RunLoop().RunUntilIdle();
-}
-
 TEST_F(ThreadTest, ProvidedMessageLoopBase) {
   Thread thread("ProvidedMessageLoopBase");
   std::unique_ptr<base::sequence_manager::internal::SequenceManagerImpl>