ART: Fix leak in Heap

Delete a mutex in the destructor.

Change-Id: I7fb8f3f9e7a30baf352f384f38520eb0dd4906eb
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index ab93142..fd2426b 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -1164,6 +1164,7 @@
   STLDeleteElements(&continuous_spaces_);
   STLDeleteElements(&discontinuous_spaces_);
   delete gc_complete_lock_;
+  delete thread_flip_lock_;
   delete pending_task_lock_;
   delete backtrace_lock_;
   if (unique_backtrace_count_.LoadRelaxed() != 0 || seen_backtrace_count_.LoadRelaxed() != 0) {