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

mojo/edk/platform/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")
 
 # Declares/defines platform-dependent things. Some things are left
 # unimplemented, and the final binary must include a separate implementation
 # (e.g., //mojo/edk/base_edk, which implements these platform-dependent things
 # using //base).
 mojo_edk_source_set("platform") {
   sources = [
     "io_thread.h",
     "message_loop.h",
     "platform_handle.cc",
     "platform_handle.h",
     "platform_handle_watcher.h",
+    "platform_pipe.h",
     "platform_shared_buffer.h",
     "random.h",
     "scoped_platform_handle.h",
     "task_runner.h",
     "thread.h",
     "thread_utils.h",
     "time_ticks.h",
   ]
 
   mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
@@ skipping 48 matching lines @@
     ":test_platform",
     "//testing/gtest",
   ]
 
   # TODO(vtl): This is a suboptimal dependency, but we have to get timeouts from
   # somewhere.
   mojo_edk_deps = [ "mojo/edk/system/test" ]
 
   mojo_sdk_deps = [ "mojo/public/cpp/system" ]
 }