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

chrome/browser/chrome_browser_main.cc

Index: chrome/browser/chrome_browser_main.cc
diff --git a/chrome/browser/chrome_browser_main.cc b/chrome/browser/chrome_browser_main.cc
index 9caaf4124250991d1c91b5228c3198feda141e74..ef8d8ccada69f44a348f08479ff86379c8f99f86 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -7,6 +7,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <map>
 #include <set>
 #include <string>
 #include <utility>
@@ -33,8 +34,10 @@
 #include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/sys_info.h"
+#include "base/task_scheduler/switches.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/threading/platform_thread.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/time/default_tick_clock.h"
 #include "base/time/time.h"
 #include "base/trace_event/trace_event.h"
@@ -312,6 +315,31 @@ void AddFirstRunNewTabs(StartupBrowserCreator* browser_creator,
 }
 #endif  // !defined(OS_ANDROID) && !defined(OS_CHROMEOS)
 
+void MaybeInitializeTaskScheduler() {
+  static constexpr char kFieldTrialName[] = "BrowserScheduler";
+  std::map<std::string, std::string> variation_params;
+  if (!variations::GetVariationParams(kFieldTrialName, &variation_params)) {
+    DCHECK(!base::CommandLine::ForCurrentProcess()->HasSwitch(
+        switches::kEnableBrowserTaskScheduler))
+        << "The Browser Task Scheduler remains disabled with "
+        << switches::kEnableBrowserTaskScheduler
+        << " because there is no available variation param for this build or "
+           " the task scheduler is disabled in chrome://flags.";
+    return;
+  }
+
+  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 (sequenced_worker_pool_param != variation_params.end() &&
+      sequenced_worker_pool_param->second == "true") {
+    base::SequencedWorkerPool::RedirectToTaskSchedulerForProcess();
+  }
+}
+
 // Returns the new local state object, guaranteed non-NULL.
 // |local_state_task_runner| must be a shutdown-blocking task runner.
 PrefService* InitializeLocalState(
@@ -1218,7 +1246,7 @@ int ChromeBrowserMainParts::PreCreateThreadsImpl() {
   //         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();