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

content/browser/browser_thread_impl.h

Index: content/browser/browser_thread_impl.h
diff --git a/content/browser/browser_thread_impl.h b/content/browser/browser_thread_impl.h
index e86f4ec907a76f9fe3aab439db527f4d94f94935..1b20c9c9cc787a311876c3df297c7f61e9496ca5 100644
--- a/content/browser/browser_thread_impl.h
+++ b/content/browser/browser_thread_impl.h
@@ -5,7 +5,8 @@
 #ifndef CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_
 #define CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_
 
-#include "base/threading/platform_thread.h"
+#include "base/memory/ref_counted.h"
+#include "base/single_thread_task_runner.h"
 #include "base/threading/thread.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/browser_thread.h"
@@ -26,9 +27,12 @@ class CONTENT_EXPORT BrowserThreadImpl : public BrowserThread,
                     base::MessageLoop* message_loop);
   ~BrowserThreadImpl() override;
 
-  bool Start();
-  bool StartWithOptions(const Options& options);
-  bool StartAndWaitForTesting();
+  // Redirects tasks posted to |identifier| to |task_runner|. |task_runner| may
+  // be null to cancel the redirection and have |identifier| enter its
+  // "shutdown" phase.
+  static void RedirectThreadIDToTaskRunner(
+      BrowserThread::ID identifier,
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
 
   static void ShutdownThreadPool();