Move Task Scheduler Initialization From ios/chrome/browser to Web

ios/chrome/browser/ios_chrome_main_parts.mm

Index: ios/chrome/browser/ios_chrome_main_parts.mm
diff --git a/ios/chrome/browser/ios_chrome_main_parts.mm b/ios/chrome/browser/ios_chrome_main_parts.mm
index e295f9fa993fc7daba857985cef140772b162181..f7e37960ec631ce981270af35835c6a1bd7ead16 100644
--- a/ios/chrome/browser/ios_chrome_main_parts.mm
+++ b/ios/chrome/browser/ios_chrome_main_parts.mm
@@ -11,7 +11,6 @@
 #include "base/memory/ptr_util.h"
 #include "base/metrics/user_metrics.h"
 #include "base/path_service.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"
@@ -25,7 +24,6 @@
 #include "components/prefs/json_pref_store.h"
 #include "components/prefs/pref_service.h"
 #include "components/rappor/rappor_service_impl.h"
-#include "components/task_scheduler_util/initialization_util.h"
 #include "components/translate/core/browser/translate_download_manager.h"
 #include "components/variations/field_trial_config/field_trial_util.h"
 #include "components/variations/service/variations_service.h"
@@ -89,6 +87,11 @@ void IOSChromeMainParts::PreMainMessageLoopStart() {
 }
 
 void IOSChromeMainParts::PreCreateThreads() {
+  // IMPORTANT
+  // Calls in this function should not post tasks or create threads as
+  // components used to handle those tasks are not yet available. This work
+  // should be deferred to PreMainMessageLoopRunImpl.
+
   base::FilePath local_state_path;
   CHECK(PathService::Get(ios::FILE_LOCAL_STATE, &local_state_path));
   scoped_refptr<base::SequencedTaskRunner> local_state_task_runner =
@@ -119,27 +122,12 @@ void IOSChromeMainParts::PreCreateThreads() {
       base::MakeUnique<base::DefaultTickClock>(),
       base::Bind(&metrics::IOSTrackingSynchronizerDelegate::Create));
 
-  // IMPORTANT
-  // Do not add anything below this line until you've verified your new code
-  // does not interfere with the critical initialization order below. Some of
-  // the calls below end up implicitly creating threads and as such new calls
-  // typically either belong before them or in a later startup phase.
-
   // Now that the command line has been mutated based on about:flags, we can
-  // initialize field trials and setup metrics. The field trials are needed by
-  // IOThread's initialization in ApplicationContext's PreCreateThreads.
+  // initialize field trials. The field trials are needed by IOThread's
+  // initialization which happens in BrowserProcess:PreCreateThreads. Metrics
+  // initialization is handled in PreMainMessageLoopRun since it posts tasks.
   SetupFieldTrials();
 
-  // Task Scheduler initialization needs to be here for the following reasons:
-  //   * After |SetupFieldTrials()|: Initialization uses variations.
-  //   * 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();
-
-
   // Initialize FieldTrialSynchronizer system.
   field_trial_synchronizer_.reset(new ios::FieldTrialSynchronizer);
 
@@ -216,13 +204,6 @@ void IOSChromeMainParts::PostMainMessageLoopRun() {
 
 void IOSChromeMainParts::PostDestroyThreads() {
   application_context_->PostDestroyThreads();
-
-  // The TaskScheduler was initialized before invoking
-  // |application_context_->PreCreateThreads()|. To maintain scoping symmetry,
-  // perform the shutdown after |application_context_->PostDestroyThreads()|.
-  base::TaskScheduler* task_scheduler = base::TaskScheduler::GetInstance();
-  if (task_scheduler)
-    task_scheduler->Shutdown();
 }
 
 // This will be called after the command-line has been mutated by about:flags