From https://codereview.chromium.org/2771413003/:

- Organize the Mojo interfaces under media/mojo/interfaces into subfolders as appropriate. Candidates for moving are media/gpu and media/remoting.

- Add a README.md explaining the folder structure, what belongs in "core media", and when it is appropriate to add a new sub-folder.

- Identify "common" media interfaces that are appropriate to share among media features.

- Add DEPS to enforce layering constraints.  E.g., "core media" should not depend on /net, although other media features/interfaces may.

- Cleanup/organize .gn targets, ipc/struct_traits and other files as appropriate.