Disallow posting tasks to SequencedWorkerPools by default.

content/browser/browser_main_loop.cc

Index: content/browser/browser_main_loop.cc
diff --git a/content/browser/browser_main_loop.cc b/content/browser/browser_main_loop.cc
index b652408781292dcabc393cb8919471f25ea4acb3..b6be6f4a04994c00610758ed1407ee3dbdac5203 100644
--- a/content/browser/browser_main_loop.cc
+++ b/content/browser/browser_main_loop.cc
@@ -31,6 +31,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "base/system_monitor/system_monitor.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/timer/hi_res_timer_manager.h"
@@ -734,6 +735,11 @@ void BrowserMainLoop::PostMainMessageLoopStart() {
 }
 
 int BrowserMainLoop::PreCreateThreads() {
+  // SequencedWorkerPool shouldn't be enabled yet. It should be enabled below by
+  // either |parts_|->PreCreateThreads() or
+  // base::SequencedWorkerPool::EnableForProcess().
+  DCHECK(!base::SequencedWorkerPool::IsEnabled());
+
   if (parts_) {
     TRACE_EVENT0("startup",
         "BrowserMainLoop::CreateThreads:PreCreateThreads");
@@ -741,6 +747,13 @@ int BrowserMainLoop::PreCreateThreads() {
     result_code_ = parts_->PreCreateThreads();
   }
 
+  // Enable SequencedWorkerPool if |parts_|->PreCreateThreads() hasn't enabled
+  // it.
+  // TODO(fdoray): Remove this once the SequencedWorkerPool to TaskScheduler
+  // redirection experiment concludes https://crbug.com/622400.
+  if (!base::SequencedWorkerPool::IsEnabled())
+    base::SequencedWorkerPool::EnableForProcess();
+
   const base::CommandLine* command_line =
       base::CommandLine::ForCurrentProcess();
   // Note that we do not initialize a new FeatureList when calling this for