Clean up accessibility enums for use in Chromium.

The goal of this change is to make it possible for Chromium to use
Blink accessibility role, state, and event enums throughout
its codebase, rather than copying to separate enums.

* Rename the enums in public/web so they're more concise, e.g.
    WebAccessibilityRoleDocumentArticle -> WebAXTypes::ArticleRole
    WebAccessibilityNotificationLoadComplete -> WebAXTypes::LoadComplete
* Add missing enum values currently used in Chromium, delete unused values.
* Rename a few enum values that were unclear or confusing.

This will require a cleanup change after Chromium switches over.

BUG=269034

Review URL: https://chromiumcodereview.appspot.com/22331005

git-svn-id: svn://svn.chromium.org/blink/trunk@156740 bbb929c8-8fbe-4397-9dbb-9b2b20218538