Add historgram for number of GC needed to collect a detached context.

src/counters.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.
 
 #ifndef V8_COUNTERS_H_
 #define V8_COUNTERS_H_
 
 #include "include/v8.h"
 #include "src/allocation.h"
 #include "src/base/platform/elapsed-timer.h"
@@ skipping 342 matching lines @@
   ~AggregatedHistogramTimerScope() { histogram_->Add(timer_.Elapsed()); }
 
  private:
   base::ElapsedTimer timer_;
   AggregatableHistogramTimer* histogram_;
 };
 
 
 #define HISTOGRAM_RANGE_LIST(HR)                                              \
   /* Generic range histograms */                                              \
+  HR(detached_context_age_in_gc, V8.DetachedContextAgeInGC, 0, 20, 21)        \
   HR(gc_idle_time_allotted_in_ms, V8.GCIdleTimeAllottedInMS, 0, 10000, 101)   \
   HR(gc_idle_time_limit_overshot, V8.GCIdleTimeLimit.Overshot, 0, 10000, 101) \
   HR(gc_idle_time_limit_undershot, V8.GCIdleTimeLimit.Undershot, 0, 10000,    \
      101)                                                                     \
   HR(code_cache_reject_reason, V8.CodeCacheRejectReason, 1, 6, 6)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                              \
   /* Garbage collection timers. */                                            \
   HT(gc_compactor, V8.GCCompactor, 10000, MILLISECOND)                        \
   HT(gc_scavenger, V8.GCScavenger, 10000, MILLISECOND)                        \
@@ skipping 418 matching lines @@
   friend class Isolate;
 
   explicit Counters(Isolate* isolate);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(Counters);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_COUNTERS_H_