[isolate] Clean up MessageHandler::ReportMessage

- Use early return
- Add exception_string read-only root

Change-Id: Iba935a4a0308d21ced2693047fdf217b7f9e62f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097884Reviewed-by: Patrick Thier <pthier@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76335}

src/execution/messages.cc

@@ -106,7 +106,11 @@ void MessageHandler::ReportMessage(Isolate* isolate, const MessageLocation* loc,
                                    Handle<JSMessageObject> message) {
   v8::Local<v8::Message> api_message_obj = v8::Utils::MessageToLocal(message);
 
-  if (api_message_obj->ErrorLevel() == v8::Isolate::kMessageError) {
+  if (api_message_obj->ErrorLevel() != v8::Isolate::kMessageError) {
+    ReportMessageNoExceptions(isolate, loc, message, v8::Local<v8::Value>());
+    return;
+  }
+
   // We are calling into embedder's code which can throw exceptions.
   // Thus we need to save current exception state, reset it to the clean one
   // and ignore scheduled exceptions callbacks can throw.
@@ -144,17 +148,13 @@ void MessageHandler::ReportMessage(Isolate* isolate, const MessageLocation* loc,
       DCHECK(isolate->has_pending_exception());
       isolate->clear_pending_exception();
       isolate->set_external_caught_exception(false);
-        stringified =
-            isolate->factory()->NewStringFromAsciiChecked("exception");
+      stringified = isolate->factory()->exception_string();
     }
     message->set_argument(*stringified);
   }
 
   v8::Local<v8::Value> api_exception_obj = v8::Utils::ToLocal(exception);
   ReportMessageNoExceptions(isolate, loc, message, api_exception_obj);
-  } else {
-    ReportMessageNoExceptions(isolate, loc, message, v8::Local<v8::Value>());
-  }
 }
 
 void MessageHandler::ReportMessageNoExceptions(

src/init/heap-symbols.h

@@ -198,13 +198,14 @@
   V(_, dot_string, ".")                                              \
   V(_, dot_switch_tag_string, ".switch_tag")                         \
   V(_, dotAll_string, "dotAll")                                      \
-  V(_, enumerable_string, "enumerable")                              \
-  V(_, element_string, "element")                                    \
   V(_, Error_string, "Error")                                        \
-  V(_, errors_string, "errors")                                      \
+  V(_, EvalError_string, "EvalError")                                \
+  V(_, element_string, "element")                                    \
+  V(_, enumerable_string, "enumerable")                              \
   V(_, error_to_string, "[object Error]")                            \
+  V(_, errors_string, "errors")                                      \
   V(_, eval_string, "eval")                                          \
-  V(_, EvalError_string, "EvalError")                                \
+  V(_, exception_string, "exception")                                \
   V(_, exec_string, "exec")                                          \
   V(_, false_string, "false")                                        \
   V(_, FinalizationRegistry_string, "FinalizationRegistry")          \