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..c1b9e7d67409fbbbf2135c18e69a917797166d9b 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 +735,8 @@ void BrowserMainLoop::PostMainMessageLoopStart() {
 }
 
 int BrowserMainLoop::PreCreateThreads() {
+  DCHECK(!base::SequencedWorkerPool::IsEnabled());

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

DCHECK are supposed to document invariants, so a DCHECK followed by an if
doesn't make sense.

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

Added comment to clarify.

SequencedWorkerPool is not enabled when BrowserMainLoop::PreCreateThreads
enters. In Chrome, it is enabled by parts_->PreCreateThreads() below. In other
products (android webview, cast...) it is enabled by
base::SequencedWorkerPool::EnableForProcess() below (surrounded by an if).

+
   if (parts_) {
     TRACE_EVENT0("startup",
         "BrowserMainLoop::CreateThreads:PreCreateThreads");
@@ -741,6 +744,13 @@ int BrowserMainLoop::PreCreateThreads() {
     result_code_ = parts_->PreCreateThreads();
   }
 
+  // Enable SequencedWorkerPool for this process if it has not already been
+  // enabled by the BrowserMainParts::PreCreateThreads() call above.
+  // 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