PPAPI: Make TrackedCallback more threadsafe

ppapi/BUILD.gn

Index: ppapi/BUILD.gn
diff --git a/ppapi/BUILD.gn b/ppapi/BUILD.gn
index 9bc9e889a142c3ef8cc095f64c0badaff66a7851..24dbad68af5f00b9c38ee06ad26bac048a36a7e3 100644
--- a/ppapi/BUILD.gn
+++ b/ppapi/BUILD.gn
@@ -6,10 +6,10 @@ 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}}",
@@ -67,6 +67,7 @@ test("ppapi_unittests") {
     "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",
@@ -77,7 +78,6 @@ test("ppapi_unittests") {
     "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",
   ]