Add support for changing roots through the root visitor callback.

Needed for copying collectors.

Change-Id: Icc4a342a57e0cfb79587edb02ef8c85e08808877
diff --git a/runtime/stack.cc b/runtime/stack.cc
index 206bff3..1715664 100644
--- a/runtime/stack.cc
+++ b/runtime/stack.cc
@@ -148,8 +148,8 @@
       const DexFile::CodeItem* code_item = MethodHelper(m).GetCodeItem();
       DCHECK(code_item != NULL) << PrettyMethod(m);  // Can't be NULL or how would we compile its instructions?
       size_t frame_size = m->GetFrameSizeInBytes();
-      return GetVReg(cur_quick_frame_, code_item, m->GetCoreSpillMask(), m->GetFpSpillMask(),
-                     frame_size, vreg);
+      return *GetVRegAddr(cur_quick_frame_, code_item, m->GetCoreSpillMask(), m->GetFpSpillMask(),
+                          frame_size, vreg);
     }
   } else {
     return cur_shadow_frame_->GetVReg(vreg);