Make Error.prototype.toString spec compliant; and fix various side-effect-free error printing metho…

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 6fb325af796284c3a7aed9c64d7122d0fc275655..6ea0e19c70f5cfa52a3e6dd70605c4b5b95cd461 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -947,10 +947,6 @@ 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);
 
@@ -1249,7 +1245,6 @@ 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_;