Remove BindStateHolder and have Bind() return a Callback<> object directly.

base/cancelable_callback.h

Index: base/cancelable_callback.h
diff --git a/base/cancelable_callback.h b/base/cancelable_callback.h
index b432f61602c87abaa58fca3549c975b860f2cd8e..4de7d12201d184d64fb2bda9d1aa5de62b466d7f 100644
--- a/base/cancelable_callback.h
+++ b/base/cancelable_callback.h
@@ -173,8 +173,7 @@ class CancelableCallback<void(A1)> {
   }
 
  private:
-  void Forward(
-      typename internal::CallbackParamTraits<A1>::ForwardType a1) const {
+  void Forward(A1 a1) const {
     callback_.Run(a1);
   }