Add heap space sizes and external memory counter to V8 statistics extension.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 72e8d62a8f24a22274df8852a17ca523e5ebee49..4b91ca43cee17984d9ffa9c4d5cf6664ce4b11f8 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1597,6 +1597,10 @@ class Heap {
     global_ic_age_ = (global_ic_age_ + 1) & SharedFunctionInfo::ICAgeBits::kMax;
   }
 
+  intptr_t amount_of_external_allocated_memory() {
+    return amount_of_external_allocated_memory_;
+  }
+
   // ObjectStats are kept in two arrays, counts and sizes. Related stats are
   // stored in a contiguous linear buffer. Stats groups are stored one after
   // another.