Reintroduce runtime zone to Isolate.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index f7d40dde32d8021e804b05dea67989ad2857cc4b..a0aecd8b275bc96c307d0e75d6700e59bb4f8b6a 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -896,6 +896,7 @@ class Isolate {
     ASSERT(handle_scope_implementer_);
     return handle_scope_implementer_;
   }
+  Zone* runtime_zone() { return &runtime_zone_; }
 
   UnicodeCache* unicode_cache() {
     return unicode_cache_;
@@ -1270,6 +1271,7 @@ class Isolate {
   v8::ImplementationUtilities::HandleScopeData handle_scope_data_;
   HandleScopeImplementer* handle_scope_implementer_;
   UnicodeCache* unicode_cache_;
+  Zone runtime_zone_;
   PreallocatedStorage in_use_list_;
   PreallocatedStorage free_list_;
   bool preallocated_storage_preallocated_;