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.

BUG=703369

Review-Url: https://codereview.chromium.org/2981083002
Cr-Commit-Position: refs/heads/master@{#489177}
15 files changed