chromite: add compiled proto files from test_platform

Enables chromite to compile test_platform's proto files for python
application. Note, this CL only appended "test_platform" to the subdirs
chromite may compile. All the files under gen/test_platform are created
automatically by `compile_build_api_proto`.

Change-Id: Iadcd517185a7dc94bc5b96c68468a0d691a72c6f
Bug:985210
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1725291
Reviewed-by: Jason Clinton <jclinton@chromium.org>
Reviewed-by: Xinan Lin <linxinan@chromium.org>
Commit-Queue: Xinan Lin <linxinan@chromium.org>
Tested-by: Xinan Lin <linxinan@chromium.org>
diff --git a/api/compile_build_api_proto.py b/api/compile_build_api_proto.py
index ff60e71..ff1049f 100644
--- a/api/compile_build_api_proto.py
+++ b/api/compile_build_api_proto.py
@@ -71,7 +71,8 @@
 
   # Only compile the subset we need for the API.
   subdirs = [os.path.join(source, 'chromite'),
-             os.path.join(source, 'chromiumos')]
+             os.path.join(source, 'chromiumos'),
+             os.path.join(source, 'test_platform')]
   for basedir in subdirs:
     for dirpath, _dirnames, filenames in os.walk(basedir):
       for filename in filenames: