Move PlatformChannelPair to //mojo/edk/platform and rename it PlatformPipe.

mojo/edk/base_edk/BUILD.gn

 # 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.
 
 import("../mojo_edk.gni")
 
 # "Completes" the implementation of //mojo/edk/platform (i.e., implements those
 # things that //mojo/edk/platform declares but doesn't define), using //base.
 mojo_edk_source_set("base_edk") {
   sources = [
     "io_thread.cc",
     "platform_handle_watcher_impl.cc",
     "platform_handle_watcher_impl.h",
     "platform_message_loop_for_io_impl.cc",
     "platform_message_loop_for_io_impl.h",
     "platform_message_loop_impl.cc",
     "platform_message_loop_impl.h",
+    "platform_pipe.cc",
     "platform_task_runner_impl.cc",
     "platform_task_runner_impl.h",
     "random.cc",
     "thread_utils.cc",
     "time_ticks.cc",
   ]
 
   deps = [
     "//base",
   ]
@@ skipping 45 matching lines @@
 # Tests //mojo/edk/platform and //mojo/edk/platform:test_platform using the
 # implementation of :platform provided by :base_edk and :test_base_edk.
 mojo_edk_unittests("mojo_edk_platform_unittests") {
   deps = [
     ":base_edk",
     ":test_base_edk",
   ]
 
   mojo_edk_deps = [ "mojo/edk/platform:unittests" ]
 }