Extend --trace-gc to output amount of memory reserved for V8 heap.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index a2702d35432df227d2ead5faa7ca3a1220603fb9..cdc693369aa65a083aae8a9f69a65433b825d409 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -2445,9 +2445,17 @@ class GCTracer BASE_EMBEDDED {
     return (static_cast<double>(HEAP->SizeOfObjects())) / MB;
   }
 
-  double start_time_;  // Timestamp set in the constructor.
-  intptr_t start_size_;  // Size of objects in heap set in constructor.
-  GarbageCollector collector_;  // Type of collector.
+  // Timestamp set in the constructor.
+  double start_time_;
+
+  // Size of objects in heap set in constructor.
+  intptr_t start_object_size_;
+
+  // Size of memory allocated from OS set in constructor.
+  intptr_t start_memory_size_;
+
+  // Type of collector.
+  GarbageCollector collector_;
 
   // A count (including this one, e.g. the first collection is 1) of the
   // number of garbage collections.