Create TaskScheduler in WebMainLoop constructor.

ios/web/app/web_main_loop.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/web/app/web_main_loop.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 53 matching lines @@
 
 // The currently-running WebMainLoop.  There can be one or zero.
 // TODO(rohitrao): Desktop uses this to implement
 // ImmediateShutdownAndExitProcess.  If we don't need that functionality, we can
 // remove this.
 WebMainLoop* g_current_web_main_loop = nullptr;
 
 WebMainLoop::WebMainLoop() : result_code_(0), created_threads_(false) {
   DCHECK(!g_current_web_main_loop);
   g_current_web_main_loop = this;
+
+  // Use an empty string as TaskScheduler name to match the suffix of browser
+  // process TaskScheduler histograms.
+  base::TaskScheduler::Create("");
 }
 
 WebMainLoop::~WebMainLoop() {
   DCHECK_EQ(this, g_current_web_main_loop);
   g_current_web_main_loop = nullptr;
 }
 
 void WebMainLoop::Init() {
   parts_.reset(web::GetWebClient()->CreateWebMainParts());
 }
@@ skipping 54 matching lines @@
 
 int WebMainLoop::PreCreateThreads() {
   if (parts_) {
     parts_->PreCreateThreads();
   }
 
   return result_code_;
 }
 
 int WebMainLoop::CreateThreads() {
-  auto task_scheduler_init_params =
-      GetWebClient()->GetTaskSchedulerInitParams();
-  if (!task_scheduler_init_params)
-    task_scheduler_init_params = GetDefaultTaskSchedulerInitParams();
-  DCHECK(task_scheduler_init_params);
-
-  base::TaskScheduler::CreateAndSetDefaultTaskScheduler(
-      "", *task_scheduler_init_params.get());
+  {
+    auto task_scheduler_init_params =
+        GetWebClient()->GetTaskSchedulerInitParams();
+    if (!task_scheduler_init_params)
+      task_scheduler_init_params = GetDefaultTaskSchedulerInitParams();
+    DCHECK(task_scheduler_init_params);
+    base::TaskScheduler::GetInstance()->Start(
+        *task_scheduler_init_params.get());
+  }
 
   GetWebClient()->PerformExperimentalTaskSchedulerRedirections();
 
   base::Thread::Options io_message_loop_options;
   io_message_loop_options.message_loop_type = base::MessageLoop::TYPE_IO;
 
   // Start threads in the order they occur in the WebThread::ID
   // enumeration, except for WebThread::UI which is the main
   // thread.
   //
@@ skipping 140 matching lines @@
   main_thread_.reset(
       new WebThreadImpl(WebThread::UI, base::MessageLoop::current()));
 }
 
 int WebMainLoop::WebThreadsStarted() {
   cookie_notification_bridge_.reset(new CookieNotificationBridge);
   return result_code_;
 }
 
 }  // namespace web