Enable native stack walking for allocation profiling

runtime/vm/tags.h

Index: runtime/vm/tags.h
diff --git a/runtime/vm/tags.h b/runtime/vm/tags.h
index 6b16be33c4d7ffac837f6e9be06a4ddec9f0e482..59f3926f559ba56b6f9566e028cc9e8db44b3258 100644
--- a/runtime/vm/tags.h
+++ b/runtime/vm/tags.h
@@ -41,9 +41,13 @@ class VMTag : public AllStatic {
     VM_TAG_LIST(DEFINE_VM_TAG_ID)
 #undef DEFINE_VM_TAG_KIND
     kNumVMTags,
+    //
+    // All tags below |kNumVMTags| are special meta-data tags and do not
+    // indicate the state of the VM during a sample.
+    //
     kRootTagId,       // Special tag used as root of all profiles.
     kTruncatedTagId,  // Special tag used to indicate a truncated call stack.
-    // ProfileInfo tags.
+    // Code transition tags (used by unit tests).
     kNoneCodeTagId,
     kOptimizedCodeTagId,
     kUnoptimizedCodeTagId,