Mojo: Add logging macros (and supporting code) for use in mojo/public/cpp.

mojo/environment/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.
 
 # GYP version: mojo.gyp:mojo_environment_chromium
 static_library("chromium") {
   output_name = "mojo_environment_chromium"
 
   sources = [
     "environment.cc",
+    # TODO(vtl): This is kind of ugly. (See TODO in logging.h.)
+    "../public/cpp/environment/logging.h",
+    "../public/cpp/environment/lib/logging.cc",
   ]
 
   deps = [
     ":chromium_impl",
     "//mojo/common",
   ]
 
   forward_dependent_configs_from = [
     ":chromium_impl",
   ]
@@ skipping 14 matching lines @@
   defines = [
     "MOJO_ENVIRONMENT_IMPL_IMPLEMENTATION",
   ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
     "//mojo/common",
   ]
 }