GitLab

1. Add AXPlatformNodeWin::GetTextAttributeValue which is used to query the
   TEXTATTRIBUTEID

2. Implement AXPlatformNodeTextRangeProviderWin::GetAttributeValue
   a) Get all anchors in the range
   b) Query the TEXTATTRIBUTEID on each anchor
     b.1) If all the same value, return that value
     b.2) If there's multiple values, return a reserved mixed value
     b.3) If none support it, return a reserved not supported value

3. Implement UIA_IsHiddenAttributeId

Bug: 847971
Change-Id: I7e315ab8ec2c01de62aa8cce51a697464424fc7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542575Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Adam Ettenberger <adettenb@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#645380}