Migrate BrowserAccessibility windows unique id handling to AXPlatformNodeWin.

Global identifiers are allocated out of ui::GetNextAXPlatformNodeUniqueId
currently. Both BrowserAccessibility and the AXPlatform code both independently
assign a unique id during object creation.

What we want to have happen is for the BrowserAccessibilityComWin class,
and it's typical owner, the BrowserAccessibilityWin, to share the same unique
id. This makes it easier to reason about as well as necessary to be able to
Implement get_uniqueID().

This CL makes private the |unique_id_| used in both above base classes private
and forces all access through virtual class methods. The BrowserAccessibilityWin
overrides and implements in terms of AXPlatformNodeWin.

Note that this landed initially but was reverted.  See:
https://codereview.chromium.org/2981083002/

BUG=703369

Change-Id: I95b26bfbb0dfd24f2048dccafecc54ee1630150a
Reviewed-on: https://chromium-review.googlesource.com/585967
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Doug Turner <dougt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490307}
15 files changed