Add a histogram timer around low memory notifications from API

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 281 matching lines @@
 };
 
 
 #define HISTOGRAM_TIMER_LIST(HT)                             \
   /* Garbage collection timers. */                           \
   HT(gc_compactor, V8.GCCompactor)                           \
   HT(gc_scavenger, V8.GCScavenger)                           \
   HT(gc_context, V8.GCContext) /* GC context cleanup time */ \
   HT(gc_idle_notification, V8.GCIdleNotification)            \
   HT(gc_incremental_marking, V8.GCIncrementalMarking)        \
+  HT(gc_low_memory_notification, V8.GCLowMemoryNotification) \
   /* Parsing timers. */                                      \
   HT(parse, V8.Parse)                                        \
   HT(parse_lazy, V8.ParseLazy)                               \
   HT(pre_parse, V8.PreParse)                                 \
   /* Total compilation times. */                             \
   HT(compile, V8.Compile)                                    \
   HT(compile_eval, V8.CompileEval)                           \
   HT(compile_lazy, V8.CompileLazy)
 
 #define HISTOGRAM_PERCENTAGE_LIST(HP)                                 \
@@ skipping 374 matching lines @@
   friend class Isolate;
 
   explicit Counters(Isolate* isolate);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(Counters);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_COUNTERS_H_