Avoid direct use of QuasiAtomic by using Atomic.

Change-Id: I92d0f822cddb2085de7cbb4c44c3a429081892a2
diff --git a/runtime/jdwp/jdwp_handler.cc b/runtime/jdwp/jdwp_handler.cc
index 05bfe0d..b9379f5 100644
--- a/runtime/jdwp/jdwp_handler.cc
+++ b/runtime/jdwp/jdwp_handler.cc
@@ -1671,7 +1671,7 @@
      * so waitForDebugger() doesn't return if we stall for a bit here.
      */
     Dbg::GoActive();
-    QuasiAtomic::Write64(&last_activity_time_ms_, 0);
+    last_activity_time_ms_.StoreSequentiallyConsistent(0);
   }
 
   /*
@@ -1751,7 +1751,7 @@
    * the initial setup.  Only update if this is a non-DDMS packet.
    */
   if (request.GetCommandSet() != kJDWPDdmCmdSet) {
-    QuasiAtomic::Write64(&last_activity_time_ms_, MilliTime());
+    last_activity_time_ms_.StoreSequentiallyConsistent(MilliTime());
   }
 
   /* tell the VM that GC is okay again */