Add PRESUMIT for accessibility_mode_enums values

Values in content/common/accessibility_mode_enums.h need to be kept in
sync with content/browser/resources/accessibility/accessibility.js. It's
easy to forget and we should have a PRESUBMIT guarding against mistakes.

BUG=692087

Review-Url: https://codereview.chromium.org/2852023002
Cr-Commit-Position: refs/heads/master@{#482174}
2 files changed