Add shell::Tracer object and wire up to --trace-startup on Android

shell/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("//build/config/ui.gni")
 import("//mojo/public/mojo.gni")
 import("//mojo/public/mojo_application.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//mojo/tools/embed/rules.gni")
 import("//testing/test.gni")
@@ skipping 114 matching lines @@
     "context.cc",
     "context.h",
     "filename_util.cc",
     "filename_util.h",
     "in_process_native_runner.cc",
     "in_process_native_runner.h",
     "out_of_process_native_runner.cc",
     "out_of_process_native_runner.h",
     "task_runners.cc",
     "task_runners.h",
+    "tracer.cc",
+    "tracer.h",
     "url_resolver.cc",
     "url_resolver.h",
   ]
 
   deps = [
     ":child_controller_bindings",
     "//base",
     "//base/third_party/dynamic_annotations",
     "//base:base_static",
     "//mojo/application",
@@ skipping 321 matching lines @@
     "//mojo/public/cpp/system:system",
     "//mojo/services/http_server/public/cpp",
     "//mojo/services/http_server/public/interfaces",
     "//mojo/services/network/public/interfaces",
     "//shell/test:bindings",
     ":embed_pingable",
   ]
 
   data_deps = [ "//services/http_server:http_server($default_toolchain)" ]
 }