Implement MSAA/IA2 HelpText property

This change implements HelpText property for views by exposing
one of the following:
1. If Placeholder is available, set its value as the HelpText.
2. If the element has NameFrom::kTitle or NameFrom::kPlaceholder,
expose StringAttribute::kName as the HelpText.
3. If Tooltip is available, set its value as the HelpText.
4. Return S_FALSE if there is no help information available.

This change also adds an AXPlatformNodeWinTest to test
the new implementation of HelpText property.

Change-Id: I8a3d0d38273da75d7969a60f70a4627c96e8ffde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1661003
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Commit-Queue: Maria Villarreal <mavill@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#670267}
3 files changed