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

mojo/edk/embedder/embedder_unittest.cc

 // Copyright 2014 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.
 
 #include "mojo/edk/embedder/embedder.h"
 
 #include <string.h>
 
 #include <memory>
 
@@ skipping 11 matching lines @@
 #include "mojo/edk/util/mutex.h"
 #include "mojo/edk/util/ref_ptr.h"
 #include "mojo/edk/util/thread_annotations.h"
 #include "mojo/edk/util/waitable_event.h"
 #include "mojo/public/c/system/core.h"
 #include "mojo/public/cpp/system/handle.h"
 #include "mojo/public/cpp/system/macros.h"
 #include "mojo/public/cpp/system/message_pipe.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
+using mojo::platform::TaskRunner;
 using mojo::system::test::TestIOThread;
 using mojo::util::ManualResetWaitableEvent;
 using mojo::util::Mutex;
 using mojo::util::MutexLocker;
 using mojo::util::RefPtr;
 
 namespace mojo {
 namespace embedder {
 namespace {
 
@@ skipping 90 matching lines @@
   MOJO_DISALLOW_COPY_AND_ASSIGN(ScopedTestChannel);
 };
 
 class EmbedderTest : public testing::Test {
  public:
   EmbedderTest() : test_io_thread_(TestIOThread::StartMode::AUTO) {}
   ~EmbedderTest() override {}
 
  protected:
   TestIOThread& test_io_thread() { return test_io_thread_; }
-  const RefPtr<PlatformTaskRunner>& test_io_task_runner() {
+  const RefPtr<TaskRunner>& test_io_task_runner() {
     return test_io_thread_.task_runner();
   }
 
  private:
   void SetUp() override { test::InitWithSimplePlatformSupport(); }
 
   void TearDown() override { EXPECT_TRUE(test::Shutdown()); }
 
   TestIOThread test_io_thread_;
 
@@ skipping 632 matching lines @@
 
   EXPECT_TRUE(test::Shutdown());
 }
 
 // TODO(vtl): Test immediate write & close.
 // TODO(vtl): Test broken-connection cases.
 
 }  // namespace
 }  // namespace embedder
 }  // namespace mojo