[wasm] Add missing mutex guard for tier down flag

R=clemensb@chromium.org

Bug: v8:10245
Change-Id: If99608d12034c241c37e8b0b345e92dce8847b9f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2064976
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66366}

src/wasm/wasm-code-manager.cc

@@ -784,6 +784,8 @@ void NativeModule::LogWasmCodes(Isolate* isolate) {
 }
 
 CompilationEnv NativeModule::CreateCompilationEnv() const {
+  // Protect concurrent accesses to {tier_down_}.
+  base::MutexGuard guard(&allocation_mutex_);
   return {module(),          use_trap_handler_, kRuntimeExceptionSupport,
           enabled_features_, kNoLowerSimd,      tier_down_};
 }