PPAPI: Make TrackedCallback more threadsafe

ppapi/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.
 
 copy("copy_test_files") {
   visibility = [ ":*" ]
   sources = [
     # Keep "test_case.html.mock-http-headers" with "test_case.html".
+    "tests/ppapi_nacl_tests_newlib.nmf",
     "tests/test_case.html",
     "tests/test_case.html.mock-http-headers",
     "tests/test_page.css",
-    "tests/ppapi_nacl_tests_newlib.nmf",
   ]
   outputs = [
     "$root_out_dir/{{source_file_part}}",
   ]
 }
 
 copy("copy_test_files2") {
   visibility = [ ":*" ]
   sources = [
     "tests/test_url_loader_data/hello.txt",
@@ skipping 37 matching lines @@
     "proxy/plugin_resource_tracker_unittest.cc",
     "proxy/plugin_var_tracker_unittest.cc",
     "proxy/ppb_var_unittest.cc",
     "proxy/ppp_instance_private_proxy_unittest.cc",
     "proxy/ppp_instance_proxy_unittest.cc",
     "proxy/ppp_messaging_proxy_unittest.cc",
     "proxy/printing_resource_unittest.cc",
     "proxy/raw_var_data_unittest.cc",
     "proxy/serialized_var_unittest.cc",
     "proxy/talk_resource_unittest.cc",
+    "proxy/tracked_callback_unittest.cc",
     "proxy/video_decoder_resource_unittest.cc",
     "proxy/video_encoder_resource_unittest.cc",
     "proxy/websocket_resource_unittest.cc",
     "shared_impl/media_stream_audio_track_shared_unittest.cc",
     "shared_impl/media_stream_buffer_manager_unittest.cc",
     "shared_impl/media_stream_video_track_shared_unittest.cc",
     "shared_impl/proxy_lock_unittest.cc",
     "shared_impl/resource_tracker_unittest.cc",
     "shared_impl/thread_aware_callback_unittest.cc",
     "shared_impl/time_conversion_unittest.cc",
-    "shared_impl/tracked_callback_unittest.cc",
     "shared_impl/var_tracker_unittest.cc",
   ]
 
   deps = [
     "//base/allocator",
     "//base/test:run_all_unittests",
     "//base/test:test_support",
     "//gpu/ipc",
     "//ipc",
     "//ipc:test_support",
@@ skipping 18 matching lines @@
   deps = [
     "//base/allocator",
     "//base/test:test_support",
     "//ppapi/proxy",
     "//ppapi/proxy:test_support",
     "//ppapi/shared_impl",
     "//ppapi/shared_impl:test_support",
     "//testing/gtest",
   ]
 }