Explicitly delete some Callback constructors to improve errors.

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index f7501f96c5d737f0a0be70590743c45e6b05329a..0b70750726ca5247ed07b7ce993659e78ef18518 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -79,6 +79,9 @@ class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
   CallbackBase(CallbackBase&& c);
   CallbackBase& operator=(CallbackBase&& c);
 
+  CallbackBase(const CallbackBase&) = delete;
+  CallbackBase& operator=(const CallbackBase&) = delete;
+
   explicit CallbackBase(const CallbackBase<CopyMode::Copyable>& c);
   CallbackBase& operator=(const CallbackBase<CopyMode::Copyable>& c);