Initial UserTag and dart:profiler library

runtime/vm/profiler.h

Index: runtime/vm/profiler.h
diff --git a/runtime/vm/profiler.h b/runtime/vm/profiler.h
index 64c60c4237197549172650c30c0a8a36cd597972..7ccff6a423a96b04e2cb32acdbe343bd4596d5b5 100644
--- a/runtime/vm/profiler.h
+++ b/runtime/vm/profiler.h
@@ -149,11 +149,19 @@ class Sample {
     vm_tag_ = tag;
   }
 
+  uword user_tag() const {
+    return user_tag_;
+  }
+  void set_user_tag(uword tag) {
+    user_tag_ = tag;
+  }
+
  private:
   int64_t timestamp_;
   ThreadId tid_;
   Isolate* isolate_;
   uword vm_tag_;
+  uword user_tag_;
   uword pcs_[kSampleFramesSize];
 };