Re-land http://codereview.chromium.org/9403039/, r124106

webkit/plugins/ppapi/npobject_var.h

Index: webkit/plugins/ppapi/npobject_var.h
diff --git a/webkit/plugins/ppapi/npobject_var.h b/webkit/plugins/ppapi/npobject_var.h
index b4e4269674e49a12e42065f1fb12c9a0378fc212..d9509df48e3c18483e87e96af628532f002e2d28 100644
--- a/webkit/plugins/ppapi/npobject_var.h
+++ b/webkit/plugins/ppapi/npobject_var.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/compiler_specific.h"
+#include "base/memory/weak_ptr.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/shared_impl/var.h"
 #include "webkit/plugins/webkit_plugins_export.h"
@@ -27,7 +28,8 @@ namespace ppapi {
 // PP_Var IDs) for each module. This allows us to track all references owned by
 // a given module and free them when the plugin exits independently of other
 // plugins that may be running at the same time.
-class NPObjectVar : public Var {
+class NPObjectVar : public Var,
+                    public base::SupportsWeakPtr<NPObjectVar> {
  public:
   // You should always use FromNPObject to create an NPObjectVar. This function
   // guarantees that we maintain the 1:1 mapping between NPObject and