mojo::Binding [1] is not copyable, but it would be useful to make it movable.

An example usage where moving it would be useful is found in this CL [2] (see line 62 on right-hand side).

[1] https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/binding.h?q=Binding&l=68&dr=CSs
[2] https://codereview.chromium.org/2244773002/diff/140001/services/video_capture/video_capture_device_factory_impl.h