[vm/tags] Ensure current tag is set to default only if it was not previously set.

The isolate creation callback might have set up current tag, so we dont' want to override it.

Fixes #61505
TEST=flutter ci

Change-Id: I18e7f60a9c41e96c9754476db17ad97d3d668949
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/449604
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/vm/thread.cc

@@ -437,11 +437,14 @@ void Thread::EnterIsolate(Isolate* isolate) {
     thread->ExitSafepoint();
   }
 
-  StackZone zone(thread);
-  HANDLESCOPE(thread);
-  if (group->tag_table() != GrowableObjectArray::null()) {
-    const UserTag& default_tag = UserTag::Handle(UserTag::DefaultTag(thread));
-    thread->set_current_tag(default_tag);
+  if (thread->current_tag() == UserTag::null()) {
+    // Set up current tag if it was not set up by the callback.
+    StackZone zone(thread);
+    HANDLESCOPE(thread);
+    if (group->tag_table() != GrowableObjectArray::null()) {
+      const UserTag& default_tag = UserTag::Handle(UserTag::DefaultTag(thread));
+      thread->set_current_tag(default_tag);
+    }
   }
 
   ASSERT(!thread->IsAtSafepoint());