media: Add presubmit for updating README when changing test files

I found some test files were added without updating the README file. So
adding this presubmit check to remind people to do so.

Here's an example of the warning when updating test files without updating
the README file:

-----------------------------------------
~/chromium/src$ git cl presubmit --upload
Running presubmit upload checks ...

** Presubmit Warnings **
When updating files in media/test/data, please also update media/test/data/README:
  media/test/data/vorbis-asd \
  media/test/data/vorbis-packet-0 \
  media/test/data/vorbis-packet-0_for_test

Presubmit checks took 1.4s to calculate.
-----------------------------------------

Bug: 818048
Change-Id: Ibc8b5d82aa7b0fd2688a01b736a57d8bf0e11e29
Reviewed-on: https://chromium-review.googlesource.com/1229497Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592149}

media/test/PRESUBMIT.py

+# Copyright 2018 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Top-level presubmit script for media/test/.
+
+See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
+for more details about the presubmit API built into depot_tools.
+"""
+
+def _CheckTestDataReadmeUpdated(input_api, output_api):
+  """
+  Checks to make sure the README file is updated when changing test files.
+  """
+  test_data_dir = input_api.os_path.join('media', 'test', 'data')
+  readme_path = input_api.os_path.join('media', 'test', 'data', 'README')
+  test_files = []
+  readme_updated = False
+  errors = []
+  for f in input_api.AffectedFiles():
+    local_path = f.LocalPath()
+    if input_api.os_path.dirname(local_path) == test_data_dir:
+      test_files.append(f)
+      if local_path == readme_path:
+        readme_updated = True
+        break
+  if test_files and not readme_updated:
+    errors.append(output_api.PresubmitPromptWarning(
+        'When updating files in ' + test_data_dir + ', please also update '
+        + readme_path + ':', test_files))
+  return errors
+
+def CheckChangeOnUpload(input_api, output_api):
+  return _CheckTestDataReadmeUpdated(input_api, output_api)