Remove overloading from MarkReferences

Change-Id: I480bacaec82f8ad7237405affd3eb43b7abe80cc
diff --git a/src/mod_union_table.cc b/src/mod_union_table.cc
index 19964af..410bf62 100644
--- a/src/mod_union_table.cc
+++ b/src/mod_union_table.cc
@@ -148,9 +148,9 @@
   MarkSweep* const mark_sweep_;
 };
 
-void ModUnionTableBitmap::MarkReferences(MarkSweep* mark_sweep) {
+void ModUnionTableBitmap::MarkReferences() {
   // Some tests have no image space, and therefore no mod-union bitmap.
-  ModUnionScanImageRootVisitor image_root_scanner(mark_sweep);
+  ModUnionScanImageRootVisitor image_root_scanner(GetMarkSweep());
   for (BitmapMap::iterator cur = bitmaps_.begin(); cur != bitmaps_.end(); ++cur) {
     const Space* space = cur->first;
     uintptr_t begin = reinterpret_cast<uintptr_t>(space->Begin());