Add //mojo/edk/platform and move platform_task_runners.h there.

mojo/edk/embedder/platform_task_runner.h

-// Copyright 2015 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.
-
-// This file provides an interface for "task runners", which are used within the
-// EDK itself.
-
-#ifndef MOJO_EDK_EMBEDDER_PLATFORM_TASK_RUNNER_H_
-#define MOJO_EDK_EMBEDDER_PLATFORM_TASK_RUNNER_H_
-
-#include "base/callback_forward.h"
-#include "mojo/edk/util/ref_counted.h"
-#include "mojo/public/cpp/system/macros.h"
-
-namespace mojo {
-namespace embedder {
-
-// Interface for "task runners", which can be used to schedule tasks to be run
-// asynchronously (possibly on a different thread). Implementations must be
-// thread-safe.
-class PlatformTaskRunner
-    : public util::RefCountedThreadSafe<PlatformTaskRunner> {
- public:
-  virtual ~PlatformTaskRunner() {}
-
-  // Posts a task to this task runner (i.e., schedule the task). The task must
-  // be run (insofar as this can be guaranteed). (This must not run the task
-  // synchronously.)
-  // TODO(vtl): Replace the |base::Closure| with |std::function<void()>|.
-  virtual void PostTask(const base::Closure& task) = 0;
-
-  // Returns true if this task runner may run tasks on the current thread, false
-  // otherwise (e.g., if this task runner only runs tasks on a different
-  // thread).
-  virtual bool RunsTasksOnCurrentThread() const = 0;
-
- protected:
-  PlatformTaskRunner() {}
-
- private:
-  MOJO_DISALLOW_COPY_AND_ASSIGN(PlatformTaskRunner);
-};
-
-}  // namespace embedder
-}  // namespace mojo
-
-#endif  // MOJO_EDK_EMBEDDER_PLATFORM_TASK_RUNNER_H_