Explicitly delete some Callback constructors to improve errors.

base/callback.h

Index: base/callback.h
diff --git a/base/callback.h b/base/callback.h
index 344acfe65cbb230abcfb74ba46c2d4bd7c8bd6f2..19a67398cc7851c01ba2e5c0dbcfb681c08fcbe1 100644
--- a/base/callback.h
+++ b/base/callback.h
@@ -117,8 +117,13 @@ class Callback<R(Args...), copy_mode, repeat_mode>
 
   template <typename OtherCallback,
             typename = typename std::enable_if<
-                internal::IsCallbackConvertible<OtherCallback, Callback>::value
-            >::type>
+                internal::IsOnceCallback<OtherCallback>::value>::type>
+  Callback(const OtherCallback&) = delete;
+
+  template <typename OtherCallback,
+            typename = typename std::enable_if<
+                internal::IsCallbackConvertible<OtherCallback,
+                                                Callback>::value>::type>
   Callback(OtherCallback other)
       : internal::CallbackBase<copy_mode>(std::move(other)) {}