Implement ITextRangeProvider::GetChildren

GetChildren retrieves a collection of all embedded objects that fall
within a text range.

More info here:
https://docs.microsoft.com/en-us/windows/desktop/api/uiautomationcore/nf-uiautomationcore-itextrangeprovider-getchildren

Unit test added to AXPlatformNodeTextRangeProviderTest.

Support was added to AXPlatformNodeDelegate and BrowserAccessibility
to retrieve the descendant nodes that are exposed to the platform API.
Unit test added for this to BrowserAccessibilityTest.

Bug: 928948
Change-Id: Id10b9ada1a8acabfea685bb0d95a4420007dc7ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529077
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644389}
10 files changed