Implement BrowserAccessibility accLocation in terms of AXPlatformNodeWin.

Here we are implementing one IAccessibility method, accLocation(), in terms of
ui::AXPlatformNodeWin. The idea is that we can forward IAccessibility calls to
ui::AXPlatformNodeWin and then slowly trim down, and maybe remove,
BrowserAccessibilityWin.

This CL adds a small helper method that allows the cast conversation of
platform_node_, which is created in AXPlatformNode, from a AXPlatformNode to
AXPlatformNodeWin. If this result is nullptr, we will fall back to using the
BrowserAccessibilityWin implementation.

BUG=703369

Review-Url: https://codereview.chromium.org/2763043002
Cr-Commit-Position: refs/heads/master@{#462978}
2 files changed