Mojo: Add embedder::PlatformSupport and a simple implementation (and a bit of plumbing).

mojo/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",
   ]
 }
@@ skipping 13 matching lines @@
     "../embedder/platform_channel_pair_win.cc",
     "../embedder/platform_channel_utils_posix.cc",
     "../embedder/platform_channel_utils_posix.h",
     "../embedder/platform_handle.cc",
     "../embedder/platform_handle.h",
     "../embedder/platform_handle_utils.h",
     "../embedder/platform_handle_utils_posix.cc",
     "../embedder/platform_handle_utils_win.cc",
     "../embedder/platform_handle_vector.h",
     "../embedder/platform_shared_buffer.h",
+    "../embedder/platform_support.h",
     "../embedder/scoped_platform_handle.h",
     "../embedder/simple_platform_shared_buffer.cc",
     "../embedder/simple_platform_shared_buffer.h",
     "../embedder/simple_platform_shared_buffer_posix.cc",
     "../embedder/simple_platform_shared_buffer_win.cc",
+    "../embedder/simple_platform_support.cc",
+    "../embedder/simple_platform_support.h",
     "channel.cc",
     "channel.h",
     "constants.h",
     "core.cc",
     "core.h",
     "data_pipe.cc",
     "data_pipe.h",
     "data_pipe_consumer_dispatcher.cc",
     "data_pipe_consumer_dispatcher.h",
     "data_pipe_producer_dispatcher.cc",
@@ skipping 54 matching lines @@
     "MOJO_SYSTEM_IMPLEMENTATION",
   ]
 
   all_dependent_configs = [ ":system_config" ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
   ]
 }