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

webkit/plugins/ppapi/host_var_tracker.h

Index: webkit/plugins/ppapi/host_var_tracker.h
diff --git a/webkit/plugins/ppapi/host_var_tracker.h b/webkit/plugins/ppapi/host_var_tracker.h
index fb743fc32a4300310b9da9cb433c16354472ed05..7bfb754080b30ae1cf57ae9e884a24b036b1fcd4 100644
--- a/webkit/plugins/ppapi/host_var_tracker.h
+++ b/webkit/plugins/ppapi/host_var_tracker.h
@@ -14,6 +14,7 @@
 #include "base/memory/linked_ptr.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_resource.h"
 #include "ppapi/shared_impl/function_group_base.h"
@@ -66,7 +67,12 @@ class HostVarTracker : public ::ppapi::VarTracker {
   virtual ::ppapi::ArrayBufferVar* CreateArrayBuffer(
       uint32 size_in_bytes) OVERRIDE;
 
-  typedef std::map<NPObject*, ::ppapi::NPObjectVar*> NPObjectToNPObjectVarMap;
+  // Clear the reference count of the given object and remove it from
+  // live_vars_.
+  void ForceReleaseNPObject(const base::WeakPtr< ::ppapi::NPObjectVar>& object);
+
+  typedef std::map<NPObject*, base::WeakPtr< ::ppapi::NPObjectVar> >
+      NPObjectToNPObjectVarMap;
 
   // Lists all known NPObjects, first indexed by the corresponding instance,
   // then by the NPObject*. This allows us to look up an NPObjectVar given