[vm/shared] Fix check for presence of dart mutator stack.

For GC purposes dart mutator thread could be identified by task kind, or if gc task is occupying the thread, by presence of scheduled isolate or (in case of isolategroup-shared thread) by presence of top_exit_frame.

Fixes dartbug.com/60705
TEST=ci

Change-Id: I09d212caf06e92a36efe1e5d397e5bb89846ced2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427981
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/vm/thread.h

@@ -593,7 +593,11 @@ class Thread : public ThreadState {
   bool HasDartMutatorStack() const {
     // The thread with dart mutator task might be temporarily
     // occupied by a gc task.
-    return IsDartMutatorThread() || scheduled_dart_mutator_isolate_ != nullptr;
+    return IsDartMutatorThread()
+           // mutator thread with isolate
+           || scheduled_dart_mutator_isolate_ != nullptr
+           // mutator thread without isolate
+           || top_exit_frame_info_ != 0;
   }
 
   // Returns the dart mutator [Isolate] this thread belongs to or nullptr.