Spin off TaskObserver into a dedicated header.

This CL moves the definition of MessageLoopCurrent::TaskObserver to
an individual header task/task_observer.h.

The motivation of this change comes from crrev.com/c/1295617.
MessageLoop is going away soon(-ish?), and Blink doesn't want to depend
on base::MessageLoop. It makes more sense to have TaskObserver in a
stand-alone header, given this current situation.

This CL does not touch any use sites of TaskObserver. The alias
MessageLoop::TaskObserver is still available, pending future mass
replacements.

Bug: 826203
Change-Id: I9e17bae3586e29cb76b23f5e247307760dac9a1f
Reviewed-on: https://chromium-review.googlesource.com/c/1298944
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603015}

base/BUILD.gn

@@ -779,6 +779,7 @@ jumbo_component("base") {
     "task/single_thread_task_runner_thread_mode.h",
     "task/task_executor.cc",
     "task/task_executor.h",
+    "task/task_observer.h",
     "task/task_scheduler/can_schedule_sequence_observer.h",
     "task/task_scheduler/delayed_task_manager.cc",
     "task/task_scheduler/delayed_task_manager.h",

base/message_loop/message_loop.h

@@ -175,6 +175,9 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate,
 
   // TODO(https://crbug.com/825327): Remove users of TaskObservers through
   // MessageLoop::current() and migrate the type back here.
+  //
+  // This alias is deprecated. Use base::TaskObserver instead.
+  // TODO(yutak): Replace all the use sites with base::TaskObserver.
   using TaskObserver = MessageLoopCurrent::TaskObserver;
 
   // These functions can only be called on the same thread that |this| is

base/message_loop/message_loop_current.h

@@ -12,6 +12,7 @@
 #include "base/message_loop/message_pump_for_ui.h"
 #include "base/pending_task.h"
 #include "base/single_thread_task_runner.h"
+#include "base/task/task_observer.h"
 #include "build/build_config.h"
 
 namespace base {
@@ -98,21 +99,9 @@ class BASE_EXPORT MessageLoopCurrent {
   // instance should replace its TaskRunner.
   void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
 
-  // A TaskObserver is an object that receives task notifications from the
-  // MessageLoop.
-  //
-  // NOTE: A TaskObserver implementation should be extremely fast!
-  class BASE_EXPORT TaskObserver {
-   public:
-    // This method is called before processing a task.
-    virtual void WillProcessTask(const PendingTask& pending_task) = 0;
-
-    // This method is called after processing a task.
-    virtual void DidProcessTask(const PendingTask& pending_task) = 0;
-
-   protected:
-    virtual ~TaskObserver() = default;
-  };
+  // This alias is deprecated. Use base::TaskObserver instead.
+  // TODO(yutak): Replace all the use sites with base::TaskObserver.
+  using TaskObserver = base::TaskObserver;
 
   // Forwards to MessageLoop::(Add|Remove)TaskObserver.
   // DEPRECATED(https://crbug.com/825327): only owners of the MessageLoop

base/task/task_observer.h

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_TASK_TASK_OBSERVER_H_
+#define BASE_TASK_TASK_OBSERVER_H_
+
+#include "base/base_export.h"
+#include "base/pending_task.h"
+
+namespace base {
+
+// A TaskObserver is an object that receives notifications about tasks being
+// processed on the thread it's associated with.
+//
+// NOTE: A TaskObserver implementation should be extremely fast!
+
+class BASE_EXPORT TaskObserver {
+ public:
+  // This method is called before processing a task.
+  virtual void WillProcessTask(const PendingTask& pending_task) = 0;
+
+  // This method is called after processing a task.
+  virtual void DidProcessTask(const PendingTask& pending_task) = 0;
+
+ protected:
+  virtual ~TaskObserver() = default;
+};
+
+}  // namespace base
+
+#endif  // BASE_TASK_TASK_OBSERVER_H_