Move another field away from android_atomic_cas.

Change-Id: If63aa2811e06ec401a601286a3bacb62a0da96ad
diff --git a/runtime/trace.h b/runtime/trace.h
index 3f5d80a..9c8d35b 100644
--- a/runtime/trace.h
+++ b/runtime/trace.h
@@ -23,6 +23,7 @@
 #include <string>
 #include <vector>
 
+#include "atomic.h"
 #include "base/macros.h"
 #include "globals.h"
 #include "instrumentation.h"
@@ -166,7 +167,7 @@
   const uint64_t start_time_;
 
   // Offset into buf_.
-  volatile int32_t cur_offset_;
+  AtomicInteger cur_offset_;
 
   // Did we overflow the buffer recording traces?
   bool overflow_;