Rename AccessibilityObject -> AXObject throughout.

Similarly with AccessibilityRenderObject -> AXRenderObject, and so on.

Rename testing/runner/AccessibilityUIElement -> WebAXObjectProxy, to better clarify exactly what it
is. In WebKit it was a more platform-specific concept;
in Blink it's literally just a proxy for WebAXObject.

This is a follow-up to this change, which
renamed the public interface:
https://chromiumcodereview.appspot.com/23726007

BUG=269034

Review URL: https://codereview.chromium.org/27165002

git-svn-id: svn://svn.chromium.org/blink/trunk@159916 bbb929c8-8fbe-4397-9dbb-9b2b20218538