Build API: Add Chromite unit tests endpoint.

BUG=chromium:958024, b:131688647
TEST=run_tests

Change-Id: I066df55c2260ea16e25784993a1ec7c1ef0ab014
Reviewed-on: https://chromium-review.googlesource.com/1590326
Commit-Ready: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
diff --git a/scripts/build_api.py b/scripts/build_api.py
index 99cf69b..eedc103 100644
--- a/scripts/build_api.py
+++ b/scripts/build_api.py
@@ -284,7 +284,7 @@
       if isinstance(field, common_pb2.Chroot):
         chroot_field_name = descriptor.name
         chroot = field
-        chroot_path = chroot.path
+        chroot_path = chroot.path or chroot_path
         chroot_args.extend(self._GetChrootArgs(chroot))
         break