[reference - do not submit] Always create four pools in TaskSchedulerImpl.

base/task_scheduler/task_scheduler.h

Index: base/task_scheduler/task_scheduler.h
diff --git a/base/task_scheduler/task_scheduler.h b/base/task_scheduler/task_scheduler.h
index 4b07c88998ba922f46914f7f88055b939e1947cc..6b137d9de511ebc9f1ad917794bf5524b267b1fd 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -15,8 +15,10 @@
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
 #include "base/task_runner.h"
+#include "base/task_scheduler/scheduler_worker_pool_params.h"
 #include "base/task_scheduler/task_traits.h"
 #include "base/time/time.h"
+#include "build/build_config.h"
 
 namespace gin {
 class V8Platform;
@@ -29,7 +31,24 @@ class Location;
 namespace base {
 
 class HistogramBase;
-class SchedulerWorkerPoolParams;
+
+struct BASE_EXPORT TaskSchedulerInitParams {
+  TaskSchedulerInitParams(
+      const SchedulerWorkerPoolParams& background_worker_pool_params_in,
+      const SchedulerWorkerPoolParams&
+          background_blocking_worker_pool_params_in,
+      const SchedulerWorkerPoolParams& foreground_worker_pool_params_in,
+      const SchedulerWorkerPoolParams&
+          foreground_blocking_worker_pool_params_in);
+
+  // Returns true if this object can be used to initialize a task scheduler.
+  bool IsValid() const;
+
+  const SchedulerWorkerPoolParams background_worker_pool_params;
+  const SchedulerWorkerPoolParams background_blocking_worker_pool_params;
+  const SchedulerWorkerPoolParams foreground_worker_pool_params;
+  const SchedulerWorkerPoolParams foreground_blocking_worker_pool_params;
+};
 
 // Interface for a task scheduler and static methods to manage the instance used
 // by the post_task.h API. Note: all base/task_scheduler users should go through
@@ -37,12 +56,6 @@ class SchedulerWorkerPoolParams;
 // which manages the process' instance.
 class BASE_EXPORT TaskScheduler {
  public:
-  // Returns the index of the worker pool in which a task with |traits| should
-  // run. This should be coded in a future-proof way: new traits should
-  // gracefully map to a default pool.
-  using WorkerPoolIndexForTraitsCallback =
-      Callback<size_t(const TaskTraits& traits)>;
-
   // Destroying a TaskScheduler is not allowed in production; it is always
   // leaked. In tests, it should only be destroyed after JoinForTesting() has
   // returned.
@@ -110,21 +123,23 @@ class BASE_EXPORT TaskScheduler {
 
 #if !defined(OS_NACL)
   // Creates and sets a task scheduler using default params. |name| is used to
-  // label threads and histograms. It should identify the component that calls
-  // this. CHECKs on failure. For tests, prefer base::test::ScopedTaskScheduler
-  // (ensures isolation).
+  // label threads and histograms. It should identify the component that creates
+  // the TaskScheduler. CHECKs on failure. For tests, prefer
+  // base::test::ScopedTaskScheduler (ensures isolation).
+  //
+  // This method is not available on NaCl because there is no way to get the
+  // number of processors on that platform.
   static void CreateAndSetSimpleTaskScheduler(const std::string& name);
 #endif  // !defined(OS_NACL)
 
-  // Creates and sets a task scheduler with custom worker pools. CHECKs on
-  // failure. |worker_pool_params_vector| describes the worker pools to create.
-  // |worker_pool_index_for_traits_callback| returns the index in |worker_pools|
-  // of the worker pool in which a task with given traits should run. For tests,
-  // prefer base::test::ScopedTaskScheduler (ensures isolation).
+  // Creates and sets a task scheduler using custom params. |name| is used to
+  // label threads and histograms. It should identify the component that creates
+  // the TaskScheduler. |init_params| is used to initialize the worker pools.
+  // CHECKs on failure. For tests, prefer base::test::ScopedTaskScheduler
+  // (ensures isolation).
   static void CreateAndSetDefaultTaskScheduler(
-      const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
-      const WorkerPoolIndexForTraitsCallback&
-          worker_pool_index_for_traits_callback);
+      const std::string& name,
+      const TaskSchedulerInitParams& init_params);
 
   // Registers |task_scheduler| to handle tasks posted through the post_task.h
   // API for this process. For tests, prefer base::test::ScopedTaskScheduler