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

ppapi/shared_impl/var.h

Index: ppapi/shared_impl/var.h
diff --git a/ppapi/shared_impl/var.h b/ppapi/shared_impl/var.h
index 988898a0b4c17476a5256479ac2da9da6c22b8e2..49b9272190f93fc4c15cd3f126288e3cbff5aa6e 100644
--- a/ppapi/shared_impl/var.h
+++ b/ppapi/shared_impl/var.h
@@ -17,6 +17,7 @@ namespace ppapi {
 class NPObjectVar;
 class ProxyObjectVar;
 class StringVar;
+class VarTracker;
 
 // Var -------------------------------------------------------------------------
 
@@ -48,6 +49,8 @@ class PPAPI_SHARED_EXPORT Var : public base::RefCounted<Var> {
   int32 GetExistingVarID() const;
 
  protected:
+  friend class VarTracker;
+
   Var();
 
   // Returns the unique ID associated with this string or object, creating it
@@ -62,6 +65,9 @@ class PPAPI_SHARED_EXPORT Var : public base::RefCounted<Var> {
   // before. This is used in cases where the ID is generated externally.
   void AssignVarID(int32 id);
 
+  // Reset the assigned object ID.
+  void ResetVarID() { var_id_ = 0; }
+
  private:
   // This will be 0 if no ID has been assigned (this happens lazily).
   int32 var_id_;