Initialize the Isolate's use_counter_callback_ member

BUG=none
R=marja@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/345363003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21975 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/isolate.cc

@@ -1480,7 +1480,8 @@ Isolate::Isolate()
       sweeper_thread_(NULL),
       num_sweeper_threads_(0),
       stress_deopt_count_(0),
-      next_optimization_id_(0) {
+      next_optimization_id_(0),
+      use_counter_callback_(NULL) {
   id_ = base::NoBarrier_AtomicIncrement(&isolate_counter_, 1);
   TRACE_ISOLATE(constructor);