Add a ChannelEndpointClient abstraction.

mojo/edk/system/BUILD.gn

 # 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.
 
 config("system_config") {
   defines = [
     # Ensures that dependent projects import the core functions on Windows.
     "MOJO_USE_SYSTEM_IMPL",
   ]
 }
 
 component("system") {
   output_name = "mojo_system_impl"
 
   sources = [
     "channel.cc",
     "channel.h",
     "channel_endpoint.cc",
     "channel_endpoint.h",
+    "channel_endpoint_client.h",
     "channel_endpoint_id.cc",
     "channel_endpoint_id.h",
     "channel_info.cc",
     "channel_info.h",
     "channel_manager.cc",
     "channel_manager.h",
     "configuration.cc",
     "configuration.h",
     "core.cc",
     "core.h",
@@ skipping 126 matching lines @@
 
   deps = [
     ":system",
     "//base",
     "//base/test:test_support",
     "//base/test:test_support_perf",
     "//mojo/edk/test:test_support",
     "//testing/gtest",
   ]
 }