GitLab

This CL allows us to override the AXPlatformNode's GetFromUniqueId
method.  In BrowserAccessibilityComWin we implement this by using the
BrowserAccessibility GetFromUniqueId.

BUG=703369

Review-Url: https://codereview.chromium.org/2894673003
Cr-Commit-Position: refs/heads/master@{#473771}