Run Traffic Annotation tests on Trybot only on modified files.

Traffic annotations test script on trybot is updated to only consider
the list of changed files.

Bug: 788035
Bug: 656607
Change-Id: Ib02f971edc65076c44f4aecf0589b30d381e00f7
Reviewed-on: https://chromium-review.googlesource.com/813635
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523761}

tools/traffic_annotation/scripts/check_annotations.py

@@ -137,6 +137,34 @@ class NetworkTrafficAnnotationChecker():
     return command.returncode
 
 
+  def GetModifiedFiles(self):
+    """Gets the list of modified files from git. Returns None if any error
+    happens."""
+
+    # List of files is extracted the same way as the following test recipe:
+    # https://cs.chromium.org/chromium/tools/depot_tools/recipes/recipe_modules/
+    # tryserver/api.py?l=66
+    args = ["git.bat"] if sys.platform == "win32" else ["git"]
+    args += ["diff", "--cached", "--name-only"]
+
+    original_path = os.getcwd()
+
+    # Change directory to src (two levels upper than build path).
+    os.chdir(os.path.join(self.build_path, "..", ".."))
+    command = subprocess.Popen(args, stdout=subprocess.PIPE,
+                               stderr=subprocess.PIPE)
+    stdout_text, stderr_text = command.communicate()
+
+    if stderr_text:
+      print("Could not run '%s' to get the list of changed files "
+            "beacuse: %s" % (" ".join(args), stderr_text))
+      os.chdir(original_path)
+      return None
+
+    os.chdir(original_path)
+    return stdout_text.splitlines()
+
+
 def main():
   parser = argparse.ArgumentParser(
       description="Traffic Annotation Auditor Presubmit checker.")
@@ -149,11 +177,24 @@ def main():
       '--limit', default=5,
       help='Limit for the maximum number of returned errors and warnings. '
            'Default value is 5, use 0 for unlimited.')
+  parser.add_argument(
+      '--complete', action='store_true',
+      help='Run the test on the complete repository. Otherwise only the '
+           'modified files are tested.')
 
   args = parser.parse_args()
 
   checker = NetworkTrafficAnnotationChecker(args.build_path)
-  return checker.CheckFiles(limit=args.limit)
+  if args.complete:
+    file_paths = None
+  else:
+    file_paths = checker.GetModifiedFiles()
+    if file_paths is None:
+      return -1
+    if len(file_paths) == 0:
+      return 0
+
+  return checker.CheckFiles(file_paths=file_paths, limit=args.limit)
 
 
 if '__main__' == __name__: