Decouple root visitors from object visitors.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 73af4cf7f71b390b7f0b46462417479273cb8dae..9088d23b114641e6dff0d40d43fcd3f0029ace37 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -417,10 +417,10 @@ char* Debug::RestoreDebug(char* storage) {
 
 int Debug::ArchiveSpacePerThread() { return 0; }
 
-void Debug::Iterate(ObjectVisitor* v) {
-  v->VisitPointer(&thread_local_.return_value_);
-  v->VisitPointer(&thread_local_.suspended_generator_);
-  v->VisitPointer(&thread_local_.ignore_step_into_function_);
+void Debug::Iterate(RootVisitor* v) {
+  v->VisitRootPointer(Root::kDebug, &thread_local_.return_value_);
+  v->VisitRootPointer(Root::kDebug, &thread_local_.suspended_generator_);
+  v->VisitRootPointer(Root::kDebug, &thread_local_.ignore_step_into_function_);
 }
 
 DebugInfoListNode::DebugInfoListNode(DebugInfo* debug_info): next_(NULL) {