[Tracing] Use TracingCategoryObserver in gc statistics

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 729 matching lines @@
 DEFINE_BOOL(concurrent_sweeping, true, "use concurrent sweeping")
 DEFINE_BOOL(parallel_compaction, true, "use parallel compaction")
 DEFINE_BOOL(parallel_pointer_update, true,
             "use parallel pointer update during compaction")
 DEFINE_BOOL(trace_incremental_marking, false,
             "trace progress of the incremental marking")
 DEFINE_BOOL(track_gc_object_stats, false,
             "track object counts and memory usage")
 DEFINE_BOOL(trace_gc_object_stats, false,
             "trace object counts and memory usage")
+DEFINE_INT(gc_stats, 0, "Used by tracing internally to enable gc statistics")

[Michael Lippautz, 2016/11/02 09:25:08]

Why int and not bool? 

[lpy, 2016/11/03 04:48:22]

We still want the flag be recognizable whether it's enabled by tracing, or
command line. So if we pass track_gc_object_stats or trace_gc_object_stats flag,
then it will set it to 0x01, tracing will set it to 0x10 when we start tracing
and reset it when we stop tracing.

I made an update in description.

 DEFINE_IMPLICATION(trace_gc_object_stats, track_gc_object_stats)
+DEFINE_VALUE_IMPLICATION(track_gc_object_stats, gc_stats, 1)
+DEFINE_VALUE_IMPLICATION(trace_gc_object_stats, gc_stats, 1)
 DEFINE_NEG_IMPLICATION(trace_gc_object_stats, incremental_marking)
 DEFINE_BOOL(track_detached_contexts, true,
             "track native contexts that are expected to be garbage collected")
 DEFINE_BOOL(trace_detached_contexts, false,
             "trace native contexts that are expected to be garbage collected")
 DEFINE_IMPLICATION(trace_detached_contexts, track_detached_contexts)
 #ifdef VERIFY_HEAP
 DEFINE_BOOL(verify_heap, false, "verify heap pointers before and after GC")
 #endif
 DEFINE_BOOL(move_object_start, true, "enable moving of object starts")
@@ skipping 465 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA