Disallow posting tasks to SequencedWorkerPools by default.

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 6e3e14b2cc96c364e574a4d97e1b5cb07c1a5512..5b553396c2a9b44f1d15ad2b5f2665cc6aa0d458 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -314,7 +314,13 @@ void AddFirstRunNewTabs(StartupBrowserCreator* browser_creator,
 }
 #endif  // !defined(OS_ANDROID) && !defined(OS_CHROMEOS)
 
-void MaybeInitializeTaskScheduler() {
+// Initializes TaskScheduler if enabled via field trial or command line flag. If
+// the function enables redirection of SequencedWorkerPool to TaskScheduler,
+// |sequenced_worker_pool_redirected| is set to true.
+void MaybeInitializeTaskScheduler(bool* sequenced_worker_pool_redirected) {

[gab, 2016/11/01 21:48:02]

Prefer return value to out-param.

[fdoray, 2016/11/02 12:56:09]

Done.

+  DCHECK(sequenced_worker_pool_redirected);
+  DCHECK(!*sequenced_worker_pool_redirected);
+
   static constexpr char kFieldTrialName[] = "BrowserScheduler";
   std::map<std::string, std::string> variation_params;
   if (!variations::GetVariationParams(kFieldTrialName, &variation_params)) {
@@ -335,7 +341,8 @@ void MaybeInitializeTaskScheduler() {
       variation_params.find("RedirectSequencedWorkerPools");
   if (sequenced_worker_pool_param != variation_params.end() &&
       sequenced_worker_pool_param->second == "true") {
-    base::SequencedWorkerPool::RedirectToTaskSchedulerForProcess();
+    base::SequencedWorkerPool::EnableWithRedirectionToTaskSchedulerForProcess();
+    *sequenced_worker_pool_redirected = true;
   }
 }
 
@@ -1229,7 +1236,8 @@ int ChromeBrowserMainParts::PreCreateThreadsImpl() {
   // IOThread's initialization which happens in BrowserProcess:PreCreateThreads.
   SetupFieldTrials();
 
-  // Task Scheduler initialization needs to be here for the following reasons:
+  // Initializing TaskScheduler and enabling SequencedWorkerPool 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.
@@ -1238,7 +1246,15 @@ 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.
-  MaybeInitializeTaskScheduler();
+  bool sequenced_worker_pool_redirected = false;
+  MaybeInitializeTaskScheduler(&sequenced_worker_pool_redirected);
+
+  // If MaybeInitializeTaskScheduler() hasn't enabled SequencedWorkerPool with
+  // redirection to TaskScheduler, enable it without redirection.
+  // TODO(fdoray): Remove this once the SequencedWorkerPool to TaskScheduler
+  // redirection experiment concludes https://crbug.com/622400.
+  if (!sequenced_worker_pool_redirected)
+    base::SequencedWorkerPool::EnableForProcess();
 
   SetupMetrics();