Handle platform_version in packages.GetTargetVersions.

BUG=chromium:1004438
TEST=manual, run_tests

Change-Id: Ibddf89dfd641e682ba7f5bd1bca08e41c6dd100e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1866028
Tested-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/packages.py b/api/controller/packages.py
index d5d58f0..f25c896 100644
--- a/api/controller/packages.py
+++ b/api/controller/packages.py
@@ -129,6 +129,7 @@
   if android_target_version:
     output_proto.android_target_version = android_target_version
   output_proto.chrome_version = packages.determine_chrome_version(build_target)
+  output_proto.platform_version = packages.determine_platform_version()
   # TODO(crbug.com/1004438): Implement remaining version fields.