Experiment with redirecting all BrowserThreads (but UI/IO) to TaskScheduler

base/threading/thread.h

Index: base/threading/thread.h
diff --git a/base/threading/thread.h b/base/threading/thread.h
index b8136db8829c172b1a791b372291af4bfffb7ec2..e63112cbbeea6684f4d118db01a2fe9117ecfa6d 100644
--- a/base/threading/thread.h
+++ b/base/threading/thread.h
@@ -244,6 +244,11 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // Bind this Thread to an existing MessageLoop instead of starting a new one.
   void SetMessageLoop(MessageLoop* message_loop);
 
+  // True only if |message_loop_| was externally provided by |SetMessageLoop()|
+  // in which case this Thread has no underlying |thread_| and should merely
+  // drop |message_loop_| on Stop().
+  bool using_external_message_loop_ = false;
+
  private:
 #if defined(OS_WIN)
   enum ComStatus {
@@ -291,11 +296,6 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   MessageLoop* message_loop_ = nullptr;
   RunLoop* run_loop_ = nullptr;
 
-  // True only if |message_loop_| was externally provided by |SetMessageLoop()|
-  // in which case this Thread has no underlying |thread_| and should merely
-  // drop |message_loop_| on Stop().
-  bool using_external_message_loop_ = false;
-
   // Stores Options::timer_slack_ until the message loop has been bound to
   // a thread.
   TimerSlack message_loop_timer_slack_ = TIMER_SLACK_NONE;