Pass paths to protos in GetCoverageRulesRequest.

- These protos can by large, and writing request to a file for
Build API can take minutes. I suspect the size of the files is
also making the Build API call hang indefinitely and eventually
kill the Swarming task:

https://luci-milo.appspot.com/raw/build/logs.chromium.org/chromeos/led/andrewlamb_google.com/0023c75cae755ff841107e84f169327b5264b548bcd0a9ff1146fcfd66b19a30/+/build.proto?server=chromeos-swarming.appspot.com

Pass a path to files containing the proto in the request, instead
of the actual proto.

- Breaking change, but these protos are not used in production
yet.

BUG=b:182898188
TEST=CQ

Change-Id: I8607daa191afc0e89241135a8d0eea55c151ce1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/3088245
Reviewed-by: Sean McAllister <smcallis@google.com>
Commit-Queue: Andrew Lamb <andrewlamb@chromium.org>
3 files changed