Ignore presubmit.py when linting layout expectations files.

This is a followup to r515890

Change-Id: Ib083a3f72e84728700fe88c439d93acdc43a9ef8
Reviewed-on: https://chromium-review.googlesource.com/765215
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515892}

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/base.py

@@ -1336,6 +1336,8 @@ class Port(object):
         for (_, _, filenames) in self._filesystem.walk(flag_path):
             if 'README.txt' in filenames:
                 filenames.remove('README.txt')
+            if 'PRESUBMIT.py' in filenames:
+                filenames.remove('PRESUBMIT.py')
             for filename in filenames:
                 path = self._filesystem.join(flag_path, filename)
                 expectations[path] = self._filesystem.read_text_file(path)