[Cast] Add DEPS for CmaBackendProxy

This CL adds DEPS for CmaBackendProxy:
- Protobuf is needed for processing of messages sent to/from gRPC (in a
  future CL)
- Openscreen is needed for proto files

Change-Id: I376015023915d4ba96a77fee6604726d3b450a72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537231Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Ryan Keane <rwkeane@google.com>
Cr-Commit-Position: refs/heads/master@{#828085}

chromecast/media/cma/backend/proxy/DEPS

+include_rules = [
+  "+third_party/openscreen/src/cast/cast_core/api",
+  "+third_party/protobuf",
+]