Add a histogram timer around low memory notifications from API

src/api.cc

 // 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.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 5045 matching lines @@
   i::Isolate* isolate = i::Isolate::Current();
   if (isolate == NULL || !isolate->IsInitialized()) return true;
   if (!i::FLAG_use_idle_notification) return true;
   return isolate->heap()->IdleNotification(hint);
 }
 
 
 void v8::V8::LowMemoryNotification() {
   i::Isolate* isolate = i::Isolate::Current();
   if (isolate == NULL || !isolate->IsInitialized()) return;
-  isolate->heap()->CollectAllAvailableGarbage("low memory notification");
+  {
+    i::HistogramTimerScope idle_notification_scope(
+        isolate->counters()->gc_low_memory_notification());
+    isolate->heap()->CollectAllAvailableGarbage("low memory notification");
+  }
 }
 
 
 int v8::V8::ContextDisposedNotification() {
   i::Isolate* isolate = i::Isolate::Current();
   if (!isolate->IsInitialized()) return 0;
   return isolate->heap()->NotifyContextDisposed();
 }
 
 
@@ skipping 1639 matching lines @@
   // Returning true tells the caller that it need not
   // continue to call IdleNotification.
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   if (!i::FLAG_use_idle_notification) return true;
   return isolate->heap()->IdleNotification(idle_time_in_ms);
 }
 
 
 void v8::Isolate::LowMemoryNotification() {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
-  isolate->heap()->CollectAllAvailableGarbage("low memory notification");
+  {
+    i::HistogramTimerScope idle_notification_scope(
+        isolate->counters()->gc_low_memory_notification());
+    isolate->heap()->CollectAllAvailableGarbage("low memory notification");
+  }
 }
 
 
 int v8::Isolate::ContextDisposedNotification() {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   return isolate->heap()->NotifyContextDisposed();
 }
 
 
 String::Utf8Value::Utf8Value(v8::Handle<v8::Value> obj)
@@ skipping 889 matching lines @@
   Isolate* isolate = reinterpret_cast<Isolate*>(info.GetIsolate());
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 } }  // namespace v8::internal