[Tracing] Implement TracingCategoryObserver.

src/v8.gyp

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 1198 matching lines @@
         'string-stream.cc',
         'string-stream.h',
         'strtod.cc',
         'strtod.h',
         'ic/stub-cache.cc',
         'ic/stub-cache.h',
         'tracing/trace-event.cc',
         'tracing/trace-event.h',
         'tracing/traced-value.cc',
         'tracing/traced-value.h',
+        'tracing/tracing-flag.cc',
+        'tracing/tracing-flag.h',
         'transitions-inl.h',
         'transitions.cc',
         'transitions.h',
         'type-feedback-vector-inl.h',
         'type-feedback-vector.cc',
         'type-feedback-vector.h',
         'type-hints.cc',
         'type-hints.h',
         'type-info.cc',
         'type-info.h',
@@ skipping 1251 matching lines @@
       'conditions': [
         ['want_separate_host_toolset_mkpeephole==1', {
           'toolsets': ['host'],
         }, {
           'toolsets': ['target'],
         }],
       ],
     },
   ],
 }