MidiScheduler binds to task runner of constructing thread

media/midi/midi_scheduler.h

Index: media/midi/midi_scheduler.h
diff --git a/media/midi/midi_scheduler.h b/media/midi/midi_scheduler.h
index 17354a96a0d3b5d6c57b6b0ee074c82f15a838b8..2d79fc8266096405761a75aa7ccc4b03c04d4bca 100644
--- a/media/midi/midi_scheduler.h
+++ b/media/midi/midi_scheduler.h
@@ -10,6 +10,8 @@
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
+#include "base/single_thread_task_runner.h"
+#include "base/threading/thread_checker.h"
 #include "media/midi/midi_export.h"
 
 namespace media {
@@ -21,12 +23,17 @@ class MidiManagerClient;
 // TODO(crbug.com/467442): Make tasks cancelable per client.
 class MIDI_EXPORT MidiScheduler final {
  public:
+  // Both constructor and destructor should be run on the same thread. The
+  // instance is bound to the TaskRunner of the constructing thread, on which
+  // InvokeClosure() is run.
   explicit MidiScheduler(MidiManager* manager);
   ~MidiScheduler();
 
-  // Post |closure| to the current message loop safely. The |closure| will not
-  // be invoked after MidiScheduler is deleted. AccumulateMidiBytesSent() of
-  // |client| is called internally.
+  // Post |closure| to |task_runner_| safely. The |closure| will not be invoked
+  // after MidiScheduler is deleted. AccumulateMidiBytesSent() of |client| is
+  // called internally. May be called on any thread, but the user should ensure
+  // this method is not called on other threads during/after MidiScheduler
+  // destruction.
   void PostSendDataTask(MidiManagerClient* client,
                         size_t length,
                         double timestamp,
@@ -39,6 +46,14 @@ class MIDI_EXPORT MidiScheduler final {
 
   // MidiManager should own the MidiScheduler and be alive longer.
   MidiManager* manager_;
+
+  // The TaskRunner of the thread on which the instance is constructed.
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+
+  // Ensures |weak_factory_| is destructed, and WeakPtrs are dereferenced on the
+  // same thread.
+  base::ThreadChecker thread_checker_;
+
   base::WeakPtrFactory<MidiScheduler> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MidiScheduler);