Linux/GTK embedder is placing C++ std::function instances within GTK objects

[jason-simmons]

Some GTK objects declared in the Linux embedder contain instances of std::function

For example, send_key_event in _FlKeyEmbedderResponder.

The _FlKeyEmbedderResponder is initialized using g_object_new. g_object_new appears to be zeroing out the entire _FlKeyEmbedderResponder struct and is not calling constructors for any C++ object members within the struct.

This will potentially fail on implementations of std::function that depend on proper construction. Specifically, in flutter/engine#49002 we tried to upgrade to a new version of the libcxx ABI and found that usage of the std::function members crashes if the constructor has not run.

@cbracken @robert-ancell

[dkwingsmt]

I opened a PR to replace this std::function with a C function and a user_data. flutter/engine#49293