Add missing field initialization in mark sweep.

Change-Id: Ied7deb35c1f1d975ef44c7b425128e0349ee44b0
diff --git a/runtime/gc/collector/mark_sweep.cc b/runtime/gc/collector/mark_sweep.cc
index 11e911c..56dc0e5 100644
--- a/runtime/gc/collector/mark_sweep.cc
+++ b/runtime/gc/collector/mark_sweep.cc
@@ -148,6 +148,7 @@
       finalizer_reference_list_(NULL),
       phantom_reference_list_(NULL),
       cleared_reference_list_(NULL),
+      live_stack_freeze_size_(0),
       gc_barrier_(new Barrier(0)),
       large_object_lock_("mark sweep large object lock", kMarkSweepLargeObjectLock),
       mark_stack_lock_("mark sweep mark stack lock", kMarkSweepMarkStackLock),