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

content/browser/browser_main_loop.h

Index: content/browser/browser_main_loop.h
diff --git a/content/browser/browser_main_loop.h b/content/browser/browser_main_loop.h
index 1464de7ced9cb6ae38c160ead17e1a935d9a6474..f32210ac27f033b6a7a8929b03b9ce27416dc2ad 100644
--- a/content/browser/browser_main_loop.h
+++ b/content/browser/browser_main_loop.h
@@ -26,10 +26,11 @@ namespace base {
 class CommandLine;
 class FilePath;
 class HighResolutionTimerManager;
+class MemoryPressureMonitor;
 class MessageLoop;
 class PowerMonitor;
+class SingleThreadTaskRunner;
 class SystemMonitor;
-class MemoryPressureMonitor;
 namespace trace_event {
 class TraceEventSystemStatsMonitor;
 }  // namespace trace_event
@@ -272,11 +273,20 @@ class CONTENT_EXPORT BrowserMainLoop {
 #endif
 
   // Members initialized in |CreateThreads()| ----------------------------------
+  // The |*_task_runner| members below are used in lieu of the |*_thread_| for
+  // thread IDs that are redirected to TaskScheduler (ref.
+  // ContentBrowserClient::RedirectNonUINonIOBrowserThreadsToTaskScheduler()).
   std::unique_ptr<BrowserProcessSubThread> db_thread_;
   std::unique_ptr<BrowserProcessSubThread> file_user_blocking_thread_;
   std::unique_ptr<BrowserProcessSubThread> file_thread_;
   std::unique_ptr<BrowserProcessSubThread> process_launcher_thread_;
   std::unique_ptr<BrowserProcessSubThread> cache_thread_;
+  scoped_refptr<base::SingleThreadTaskRunner> db_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> file_user_blocking_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> file_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> process_launcher_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> cache_task_runner_;
+  // The IO thread is never redirected to TaskScheduler at this point.
   std::unique_ptr<BrowserProcessSubThread> io_thread_;
 
   // Members initialized in |BrowserThreadsStarted()| --------------------------