Remove `Runnable` concept from base/bind_internal.h

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index 3e8e10f75c0b8a43b19bb61d4d5319b3211c6bf9..0fe0b2d9e198716690365dc8d6a4c82508a79188 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -64,6 +64,7 @@ class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
 
   // 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();