[objects] Skip acquiring main thread's string lock via local heap

This has similar performance as the thread_id check. It removes the need
to get the isolate for this check though, so it's a net positive.

Bug: v8:7790
Change-Id: I329bb9d5fe695fb9ef4990ba3593703b1eedca0d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2546690Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71259}

src/objects/string-inl.h

@@ -41,6 +41,10 @@ class SharedStringAccessGuardIfNeeded {
   }
 
   static bool IsNeeded(String str, Isolate** out_isolate = nullptr) {
+    LocalHeap* local_heap = LocalHeap::Current();
+    // Don't acquire the lock for the main thread.
+    if (!local_heap || local_heap->is_main_thread()) return false;
+
     Isolate* isolate;
     if (!GetIsolateFromHeapObject(str, &isolate)) {
       // If we can't get the isolate from the String, it must be read-only.
@@ -48,7 +52,7 @@ class SharedStringAccessGuardIfNeeded {
       return false;
     }
     if (out_isolate) *out_isolate = isolate;
-    return ThreadId::Current() != isolate->thread_id();
+    return true;
   }
 
  private: