Reland "Create sandbox::SeatbeltExtension to dynamically expand the Mac sandbox policy."

This is a reland of fa1f0b26ababc0ee9156619d25aa7adb39bd706d
Original change's description:
> Create sandbox::SeatbeltExtension to dynamically expand the Mac sandbox policy.
> 
> Sandbox policy extensions can be issued for resources in a trusted process,
> which generates a token that may be passed over Mojo IPC. The receiving process
> can then consume the token to be granted access to the resource. Currently only
> file-read extensions are permitted.
> 
> Bug: 510604
> Change-Id: Iaa92c48c120e051c5df72602d09329ebe84ea317
> Reviewed-on: https://chromium-review.googlesource.com/685583
> Reviewed-by: Greg Kerr <kerrnel@chromium.org>
> Reviewed-by: Ken Rockot <rockot@chromium.org>
> Reviewed-by: Justin Schuh <jschuh@chromium.org>
> Commit-Queue: Robert Sesek <rsesek@chromium.org>
> Commit-Queue: Justin Schuh <jschuh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#505819}

Tbr: Ken Rockot <rockot@chromium.org>
Bug: 510604
Change-Id: I594e3f976de121f7fb870474ec6ea795299c9d21
Reviewed-on: https://chromium-review.googlesource.com/697764
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506124}

• mojo/public/tools/bindings/chromium_bindings_configuration.gni[diff]
• sandbox/BUILD.gn[diff]
• sandbox/mac/BUILD.gn[diff]
• sandbox/mac/mojom/BUILD.gn[Added - diff]
• sandbox/mac/mojom/DEPS[Added - diff]
• sandbox/mac/mojom/OWNERS[Added - diff]
• sandbox/mac/mojom/seatbelt_extension_token.mojom[Added - diff]
• sandbox/mac/mojom/seatbelt_extension_token.typemap[Added - diff]
• sandbox/mac/mojom/seatbelt_extension_token_struct_traits.cc[Added - diff]
• sandbox/mac/mojom/seatbelt_extension_token_struct_traits.h[Added - diff]
• sandbox/mac/mojom/struct_traits_unittest.cc[Added - diff]
• sandbox/mac/mojom/traits_test_service.mojom[Added - diff]
• sandbox/mac/mojom/typemaps.gni[Added - diff]
• sandbox/mac/seatbelt_extension.cc[Added - diff]
• sandbox/mac/seatbelt_extension.h[Added - diff]
• sandbox/mac/seatbelt_extension_token.cc[Added - diff]
• sandbox/mac/seatbelt_extension_token.h[Added - diff]
• sandbox/mac/seatbelt_extension_unittest.cc[Added - diff]