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

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 d80cd693e64aeda170aad56704d3c5d47f434f6d..1363f9f566eb66449274570a5b890df084c74fa1 100644
--- a/ios/chrome/browser/ios_chrome_main_parts.mm
+++ b/ios/chrome/browser/ios_chrome_main_parts.mm
@@ -11,6 +11,7 @@
 #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"
@@ -59,6 +60,43 @@
 #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) {
@@ -134,7 +172,7 @@ void IOSChromeMainParts::PreCreateThreads() {
   //         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();