[M56] Revert "Enable Initialization of the Task Scheduler by Default in the Browser Process"

ios/chrome/browser/ios_chrome_main_parts.mm

 // Copyright 2016 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/chrome/browser/ios_chrome_main_parts.h"
 
 #include "base/base_switches.h"
 #include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/user_metrics.h"
 #include "base/path_service.h"
+#include "base/task_scheduler/switches.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/time/default_tick_clock.h"
 #include "components/content_settings/core/browser/cookie_settings.h"
 #include "components/content_settings/core/common/content_settings_pattern.h"
 #include "components/flags_ui/pref_service_flags_storage.h"
 #include "components/language_usage_metrics/language_usage_metrics.h"
 #include "components/metrics/metrics_service.h"
 #include "components/metrics/profiler/ios/ios_tracking_synchronizer_delegate.h"
 #include "components/metrics/profiler/tracking_synchronizer.h"
 #include "components/metrics_services_manager/metrics_services_manager.h"
@@ skipping 28 matching lines @@
 #include "net/http/http_stream_factory.h"
 #include "net/url_request/url_request.h"
 #include "ui/base/l10n/l10n_util_mac.h"
 #include "ui/base/resource/resource_bundle.h"
 
 #if defined(ENABLE_RLZ)
 #include "components/rlz/rlz_tracker.h"                        // nogncheck
 #include "ios/chrome/browser/rlz/rlz_tracker_delegate_impl.h"  // nogncheck
 #endif
 
+namespace {
+
+void MaybeInitializeTaskScheduler() {
+  static constexpr char kFieldTrialName[] = "BrowserScheduler";
+  std::map<std::string, std::string> variation_params;
+  bool used_default_config = false;
+  if (!variations::GetVariationParams(kFieldTrialName, &variation_params)) {
+    if (!base::CommandLine::ForCurrentProcess()->HasSwitch(
+            switches::kEnableBrowserTaskScheduler)) {
+      return;
+    }
+
+    // TODO(robliao): Remove below once iOS uses fieldtrial_testing_config.json.
+    // Synchronize the below from fieldtrial_testing_config.json.
+    DCHECK(variation_params.empty());
+    variation_params["Background"] = "3;8;0.1;0;30000";
+    variation_params["BackgroundFileIO"] = "3;8;0.1;0;30000";
+    variation_params["Foreground"] = "8;32;0.3;0;30000";
+    variation_params["ForegroundFileIO"] = "8;32;0.3;0;30000";
+    used_default_config = true;
+  }
+
+  if (!task_scheduler_util::InitializeDefaultTaskScheduler(variation_params))
+    return;
+
+  // TODO(gab): Remove this when http://crbug.com/622400 concludes.
+  const auto sequenced_worker_pool_param =
+      variation_params.find("RedirectSequencedWorkerPools");
+  if (used_default_config ||
+      (sequenced_worker_pool_param != variation_params.end() &&
+       sequenced_worker_pool_param->second == "true")) {
+    base::SequencedWorkerPool::RedirectToTaskSchedulerForProcess();
+  }
+}
+
+}  // namespace
+
 IOSChromeMainParts::IOSChromeMainParts(
     const base::CommandLine& parsed_command_line)
     : parsed_command_line_(parsed_command_line), local_state_(nullptr) {
   // Chrome disallows cookies by default. All code paths that want to use
   // cookies need to go through one of Chrome's URLRequestContexts which have
   // a ChromeNetworkDelegate attached that selectively allows cookies again.
   net::URLRequest::SetDefaultCookiePolicyToBlock();
 }
 
 IOSChromeMainParts::~IOSChromeMainParts() {}
@@ skipping 55 matching lines @@
 
   // Task Scheduler initialization needs to be here for the following reasons:
   //   * After |SetupFieldTrials()|: Initialization uses variations.
   //   * Before |SetupMetrics()|: |SetupMetrics()| uses the blocking pool. The
   //         Task Scheduler must do any necessary redirection before then.
   //   * Near the end of |PreCreateThreads()|: The TaskScheduler needs to be
   //         created before any other threads are (by contract) but it creates
   //         threads itself so instantiating it earlier is also incorrect.
   // To maintain scoping symmetry, if this line is moved, the corresponding
   // shutdown call may also need to be moved.
-  task_scheduler_util::InitializeDefaultBrowserTaskScheduler();
+  MaybeInitializeTaskScheduler();
 
   SetupMetrics();
 
   // Initialize FieldTrialSynchronizer system.
   field_trial_synchronizer_.reset(new ios::FieldTrialSynchronizer);
 
   application_context_->PreCreateThreads();
 }
 
 void IOSChromeMainParts::PreMainMessageLoopRun() {
@@ skipping 147 matching lines @@
 
 void IOSChromeMainParts::StartMetricsRecording() {
   bool wifiOnly = local_state_->GetBoolean(prefs::kMetricsReportingWifiOnly);
   bool isConnectionCellular = net::NetworkChangeNotifier::IsConnectionCellular(
       net::NetworkChangeNotifier::GetConnectionType());
   bool mayUpload = !wifiOnly || !isConnectionCellular;
 
   application_context_->GetMetricsServicesManager()->UpdateUploadPermissions(
       mayUpload);
 }