Add tracing service and make the shell+sky_viewer+services talk to it

mojo/common/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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
 # GYP version: mojo/mojo_base.gyp:mojo_common_lib
 component("common") {
   output_name = "mojo_common_lib"
 
@@ skipping 49 matching lines @@
   ]
 }
 
 source_set("tracing_impl") {
   sources = [
     "tracing_impl.cc",
     "tracing_impl.h",
   ]
 
   deps = [
-    ":tracing_bindings",
     "//base",
+    "//mojo/public/cpp/application",
     "//mojo/public/cpp/bindings",
+    "//services/tracing:bindings",
   ]
 }
-
-mojom("tracing_bindings") {
-  sources = [ "tracing.mojom" ]
-}