Makes NativeViewport send OnBoundsChanged() after widget available

mojo/public/cpp/bindings/callback.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
 $var MAX_ARITY = 7
 
 // Copyright 2014 The Chromium Authors. All rights reserved.
@@ skipping 45 matching lines @@
       sink_->Run($if ARITY > 1 [[
 
           ]]$for ARG ,
           [[internal::Forward(a$(ARG))]]);
   }
 
   bool is_null() const {
     return !sink_.get();
   }
 
+  void reset() {
+    sink_.reset();
+  }
+
  private:
   template <typename Sink>
   struct Adapter : public Runnable {
     explicit Adapter(const Sink& sink) : sink(sink) {}
     virtual void Run($if ARITY > 0 [[
 
         ]]$for ARG ,
         [[typename internal::Callback_ParamTraits<A$(ARG)>::ForwardType a$(ARG)]]) const MOJO_OVERRIDE {
       sink.Run($if ARITY > 1 [[
 
           ]]$for ARG ,
           [[internal::Forward(a$(ARG))]]);
     }
     Sink sink;
   };
 
   internal::SharedPtr<Runnable> sink_;
 };
 
 ]]  $$ for ARITY
 
 typedef Callback<void()> Closure;
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_CALLBACK_H_