Move IA2 Role handling to AXPlatformNodeWin.

This CL moves all ia2_role handling from BrowserAccessibilityComWin to
AXPlatformNodeWin. In addition, we now forward IAccessible2::role() to
AXPlatformNodeWin as well.

BUG=703369

Review-Url: https://codereview.chromium.org/2965873002
Cr-Commit-Position: refs/heads/master@{#485872}
4 files changed