Add root types and thread id to root visiting.

Enables us to pass the root type and thread id to hprof.

Bug: 12680863
Change-Id: I6a0f1f9e3aa8f9b4033d695818ae7ca3460d67cb
diff --git a/runtime/gc/accounting/heap_bitmap.cc b/runtime/gc/accounting/heap_bitmap.cc
index c520ee6..f94cf24 100644
--- a/runtime/gc/accounting/heap_bitmap.cc
+++ b/runtime/gc/accounting/heap_bitmap.cc
@@ -72,12 +72,10 @@
   discontinuous_space_sets_.erase(it);
 }
 
-void HeapBitmap::Walk(SpaceBitmap::Callback* callback, void* arg) {
+void HeapBitmap::Walk(ObjectCallback* callback, void* arg) {
   for (const auto& bitmap : continuous_space_bitmaps_) {
     bitmap->Walk(callback, arg);
   }
-
-  DCHECK(!discontinuous_space_sets_.empty());
   for (const auto& space_set : discontinuous_space_sets_) {
     space_set->Walk(callback, arg);
   }