Rewrite Error.prototype.toString in C++.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 56bfff57380a5e3a6fba2dc662f639f6481814f5..3eb21fcee6a1eceaa68bc86938546a7ab6dfbf76 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -23,6 +23,7 @@
 #include "src/handles.h"
 #include "src/hashmap.h"
 #include "src/heap/heap.h"
+#include "src/messages.h"
 #include "src/optimizing-compile-dispatcher.h"
 #include "src/regexp-stack.h"
 #include "src/runtime/runtime.h"
@@ -994,6 +995,10 @@ class Isolate {
     date_cache_ = date_cache;
   }
 
+  ErrorToStringHelper* error_tostring_helper() {
+    return &error_tostring_helper_;
+  }
+
   Map* get_initial_js_array_map(ElementsKind kind,
                                 Strength strength = Strength::WEAK);
 
@@ -1296,6 +1301,7 @@ class Isolate {
       regexp_macro_assembler_canonicalize_;
   RegExpStack* regexp_stack_;
   DateCache* date_cache_;
+  ErrorToStringHelper error_tostring_helper_;
   unibrow::Mapping<unibrow::Ecma262Canonicalize> interp_canonicalize_mapping_;
   CallInterfaceDescriptorData* call_descriptor_data_;
   base::RandomNumberGenerator* random_number_generator_;