[wasm] Add priority to V8's --wasm-dynamic-tiering

With the existing code, V8's --wasm-dynamic-tiering flag gets ignored
in Chrome, because a Chrome flag would override it. With this CL,
the flag --wasm-dynamic-tiering can be used to enable dynamic tiering
explicitly.

R=jkummerow@chromium.org

Bug: v8:12281
Change-Id: I83b8417527872a1a920fcb543932915caf1ed754
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3322766Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78304}

src/execution/isolate.cc

@@ -2706,16 +2706,15 @@ bool Isolate::AreWasmExceptionsEnabled(Handle<Context> context) {
 
 bool Isolate::IsWasmDynamicTieringEnabled() {
 #if V8_ENABLE_WEBASSEMBLY
+  if (FLAG_wasm_dynamic_tiering) return true;
   if (wasm_dynamic_tiering_enabled_callback()) {
     HandleScope handle_scope(this);
     v8::Local<v8::Context> api_context =
         v8::Utils::ToLocal(handle(context(), this));
     return wasm_dynamic_tiering_enabled_callback()(api_context);
   }
-  return FLAG_wasm_dynamic_tiering;
-#else
-  return false;
 #endif  // V8_ENABLE_WEBASSEMBLY
+  return false;
 }
 
 Handle<Context> Isolate::GetIncumbentContext() {