[Tracing] Implement TracingCategoryObserver.

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/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
@@ skipping 1659 matching lines @@
     "src/string-builder.h",
     "src/string-search.h",
     "src/string-stream.cc",
     "src/string-stream.h",
     "src/strtod.cc",
     "src/strtod.h",
     "src/tracing/trace-event.cc",
     "src/tracing/trace-event.h",
     "src/tracing/traced-value.cc",
     "src/tracing/traced-value.h",
+    "src/tracing/tracing-flag.cc",
+    "src/tracing/tracing-flag.h",
     "src/transitions-inl.h",
     "src/transitions.cc",
     "src/transitions.h",
     "src/type-feedback-vector-inl.h",
     "src/type-feedback-vector.cc",
     "src/type-feedback-vector.h",
     "src/type-hints.cc",
     "src/type-hints.h",
     "src/type-info.cc",
     "src/type-info.h",
@@ skipping 1274 matching lines @@
   ]
 
   configs = [
     ":external_config",
     ":internal_config_base",
   ]
 }
 
 v8_fuzzer("wasm_data_section_fuzzer") {
 }