Add presubmit check for relnotes field in accessibility changes.

R=akihiroota,dmazzoni,jam,dpranke

Change-Id: I37d5035967941da1d2e3d671426133668b87f0f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2162427
Commit-Queue: Chris Hall <chrishall@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#764594}

diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index 7549d3c..8318028e 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+from collections import defaultdict
 import fnmatch
 import json
 import os
@@ -240,6 +241,7 @@
 
   def __init__(self, changed_files):
     self._changed_files = changed_files
+    self.footers = defaultdict(list)
 
   def LocalPaths(self):
     return self._changed_files
@@ -247,3 +249,7 @@
   def AffectedFiles(self, include_dirs=False, include_deletes=True,
                     file_filter=None):
     return self._changed_files
+
+  def GitFootersFromDescription(self):
+    return self.footers
+