Move vm_tags from isolate to thread, since we may have multiple threads in same isolate (GC, backgr…

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 5ab9fccead4515fafd4411cdbc3befc42420c1e4..238f26ef433309e628056f379207c4b49032cb1a 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -238,16 +238,6 @@ class Isolate : public BaseIsolate {
     mutator_thread_->set_top_exit_frame_info(value);
   }
 
-  uword vm_tag() const {
-    return vm_tag_;
-  }
-  void set_vm_tag(uword tag) {
-    vm_tag_ = tag;
-  }
-  static intptr_t vm_tag_offset() {
-    return OFFSET_OF(Isolate, vm_tag_);
-  }
-
   ApiState* api_state() const { return api_state_; }
   void set_api_state(ApiState* value) { api_state_ = value; }