Make BrowserAccessibility implement AXPlatformNodeDelegete.

As part of the fefactoring of BrowserAccessibility, we want to end up with
BrowserAccessibility inheriting from AXPlatformNodeDelegate and owning a
AXPlatformNode. This change has BrowserAccessibility implmenting a
AXPlatformNodeDelegete.  This implementation should do nothing.

BUG=703369

Review-Url: https://codereview.chromium.org/2759113004
Cr-Commit-Position: refs/heads/master@{#459511}
2 files changed