Restructure public side of content_handler service.

This CL goes from this:
  //mojo/services/public/interfaces/content_handler

to this:
  //mojo/services/content_handler/public/interfaces

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

TBR=beng

Review URL: https://codereview.chromium.org/793243002