Network annotations: Check files for removed annotations

The traffic_annotation_auditor treats the lack of output from the
traffic_annotation_extractor as nothing to check. This means some
CLs skip the CQ check, if they only remove annotations. It should still
check the annotations for removed annotations if the
extractor successfully ran.

Change-Id: I47536e1b706efd8f391448a599b2e1d1c87a3400
Bug: 1001108
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804330Reviewed-by: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Commit-Queue: Yann Dago <ydago@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697340}

tools/traffic_annotation/scripts/extractor.py

@@ -247,6 +247,11 @@ def main():
   for annotation in annotation_definitions:
     print(annotation.clang_tool_output_string())
 
+  # If all files were successfully checked for annotations but none of them had
+  # any, print something so that the traffic_annotation_auditor knows there was
+  # no error so that the files get checked for deleted annotations.
+  if not annotation_definitions:
+      print('No annotations in these files.')
   return 0