build api: add autotest HW Test tarball endpoint

BUG=chromium:923462
TEST=new tests, run_tests
TEST=manual run (see example json in proto)

Change-Id: I4e65f541ee750145e6eb47bc58fd427c6fe14b5c
Reviewed-on: https://chromium-review.googlesource.com/1427351
Commit-Ready: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Lann Martin <lannm@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
diff --git a/api/build_api.py b/api/build_api.py
index 6b84099..684a093 100644
--- a/api/build_api.py
+++ b/api/build_api.py
@@ -16,6 +16,7 @@
 
 from chromite.api import service
 from chromite.api.gen import build_api_pb2
+from chromite.api.gen import autotest_pb2
 from chromite.lib import commandline
 from chromite.lib import osutils
 
@@ -207,7 +208,7 @@
   Args:
     router (Router): The router.
   """
-  pass
+  router.Register(autotest_pb2)
 
 
 def main(argv):