Add 'flog' service implementation.

mojo/services/flog/cpp/BUILD.gn

 # Copyright 2016 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("//build/module_args/mojo.gni")
 import("$mojo_sdk_root/mojo/public/mojo_sdk.gni")
 
 mojo_sdk_source_set("cpp") {
+  restrict_external_deps = false
   sources = [
     "flog.cc",
     "flog.h",
   ]
 
   deps = [
     "$mojo_sdk_root/mojo/public/cpp/application",
     "$mojo_sdk_root/mojo/public/cpp/bindings",
     "$mojo_sdk_root/mojo/public/cpp/environment",
     "$mojo_sdk_root/mojo/services/flog/interfaces",
   ]
 }