Expose a lower bound of malloc'd memory via heap statistics

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 44e5ec0658f2b424cc6c37938a1d9ba1a3bf1aee..3336bd0c401997201820c7e5af0e9913bf1b839f 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -1816,6 +1816,8 @@ Isolate::Isolate(bool enable_serializer)
       descriptor_lookup_cache_(NULL),
       handle_scope_implementer_(NULL),
       unicode_cache_(NULL),
+      runtime_zone_(&allocator_),
+      interface_descriptor_zone_(&allocator_),
       inner_pointer_to_code_cache_(NULL),
       global_handles_(NULL),
       eternal_handles_(NULL),