[Mac] Ensure the MIDI task runners are backed by CFRunLoop.

Currently the code assumes that a TYPE_DEFAULT MesssageLoop will be backed by
CFRunLoop, which is required to receive the CoreMIDI callbacks. In the future,
that assumption may not be true, so explicitly request a TYPE_UI (CFRunLoop)
pump.

Bug: 753544
Change-Id: I5464e1299d8cb99d6fbf37d8b342417487117744
Reviewed-on: https://chromium-review.googlesource.com/682834
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504448}

media/midi/task_service.cc

@@ -6,6 +6,7 @@
 
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_task_runner_handle.h"
+#include "build/build_config.h"
 
 namespace midi {
 
@@ -139,10 +140,13 @@ scoped_refptr<base::SingleThreadTaskRunner> TaskService::GetTaskRunner(
   if (!threads_[thread]) {
     threads_[thread] = base::MakeUnique<base::Thread>(
         base::StringPrintf("MidiService_TaskService_Thread(%zu)", runner_id));
+    base::Thread::Options options;
 #if defined(OS_WIN)
     threads_[thread]->init_com_with_mta(true);
+#elif defined(OS_MACOSX)
+    options.message_loop_type = base::MessageLoop::TYPE_UI;
 #endif
-    threads_[thread]->Start();
+    threads_[thread]->StartWithOptions(options);
   }
   return threads_[thread]->task_runner();
 }