Enable iterating over roots from helper threads by moving stack walking.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index fcbf9c483775f350396dc4110804f6367429e2d0..e0f8752c1d1ec4c1c163e4bcfd46bf6fec605b11 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1598,14 +1598,6 @@ void Isolate::VisitObjectPointers(ObjectPointerVisitor* visitor,
   // Visit objects in isolate specific handles area.
   reusable_handles_.VisitObjectPointers(visitor);
 
-  // Iterate over all the stack frames and visit objects on the stack.
-  StackFrameIterator frames_iterator(validate_frames);

[Ivan Posva, 2015/08/11 17:41:31]

Can you please explain how we are validating frames after this change?

-  StackFrame* frame = frames_iterator.NextFrame();
-  while (frame != NULL) {
-    frame->VisitObjectPointers(visitor);
-    frame = frames_iterator.NextFrame();
-  }
-
   // Visit the dart api state for all local and persistent handles.
   if (api_state() != NULL) {
     api_state()->VisitObjectPointers(visitor, visit_prologue_weak_handles);