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..42c9d2f79844e4794c1279863b241895d9554e04 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -335,7 +335,7 @@ 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();
   }
 }
 
@@ -1229,7 +1229,8 @@ 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.
@@ -1240,6 +1241,12 @@ int ChromeBrowserMainParts::PreCreateThreadsImpl() {
   // shutdown call may also need to be moved.
   MaybeInitializeTaskScheduler();
 
+  // 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.
+  base::SequencedWorkerPool::EnableForProcess();

[gab, 2016/11/01 16:09:48]

Add a bool to MaybeInitializeTaskScheduler() and make this conditional (then you
can DCHECK in its impl -- I don't like that it no-ops when already enabled some
other way)

[fdoray, 2016/11/01 20:40:28]

Done.

+
   SetupMetrics();
 
   // ChromeOS needs ResourceBundle::InitSharedInstance to be called before this.