Restructure public side of native_viewport service.

This CL goes from this:
  //mojo/services/public/cpp/native_viewport
  //mojo/services/public/interfaces/native_viewport

to this:
  //mojo/services/native_viewport/public/cpp
  //mojo/services/native_viewport/public/interfaces

This CL also makes the Mojo-side changes required to roll this change into
Chromium.

TBR=beng

Review URL: https://codereview.chromium.org/795723002