Pepper: Var keeps invalid var_id if VarTracker release it and there is another reference

ppapi/shared_impl/var_tracker.cc

Index: ppapi/shared_impl/var_tracker.cc
diff --git a/ppapi/shared_impl/var_tracker.cc b/ppapi/shared_impl/var_tracker.cc
index 7f7157e7954af162a29a72f183d8c97d5368e564..d95a759069169bdb2a44fe384b6b74254efcd3e9 100644
--- a/ppapi/shared_impl/var_tracker.cc
+++ b/ppapi/shared_impl/var_tracker.cc
@@ -100,6 +100,7 @@ bool VarTracker::ReleaseVar(int32 var_id) {
     } else {
       // All other var types can just be released.
       DCHECK(info.track_with_no_reference_count == 0);
+      info.var->ResetVarID();
       live_vars_.erase(found);
     }
   }