Remove `Runnable` concept from base/bind_internal.h

base/callback_internal.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file contains utility functions and classes that help the
 // implementation, and management of the Callback objects.
 
 #ifndef BASE_CALLBACK_INTERNAL_H_
 #define BASE_CALLBACK_INTERNAL_H_
 
@@ skipping 46 matching lines @@
 // CallbackBase<MoveOnly> is a direct base class of MoveOnly callbacks, and
 // CallbackBase<Copyable> uses CallbackBase<MoveOnly> for its implementation.
 template <>
 class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
  public:
   CallbackBase(CallbackBase&& c);
   CallbackBase& operator=(CallbackBase&& c);
 
   // Returns true if Callback is null (doesn't refer to anything).
   bool is_null() const { return bind_state_.get() == NULL; }
+  explicit operator bool() const { return !is_null(); }
 
   // Returns the Callback into an uninitialized state.
   void Reset();
 
  protected:
   // In C++, it is safe to cast function pointers to function pointers of
   // another type. It is not okay to use void*. We create a InvokeFuncStorage
   // that that can store our function pointer, and then cast it back to
   // the original type on usage.
   using InvokeFuncStorage = void(*)();
@@ skipping 31 matching lines @@
   ~CallbackBase() {}
 };
 
 extern template class CallbackBase<CopyMode::MoveOnly>;
 extern template class CallbackBase<CopyMode::Copyable>;
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_