AXPlatformNodeWin implements IAccessibility. Many of these MSAA methods take a
var_id parameter indicating that the operation should be performed on a
particular child ID, rather than the called object.

This CL mimics the approach taken by BrowserAccessibilityWin.

BUG=703369

Review-Url: https://codereview.chromium.org/2806093002
Cr-Commit-Position: refs/heads/master@{#465288}
2 files changed