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ARIA trees were previously un-navigable with VoiceOver on macOS. This
was because it didn't properly fulfill the NSAccessibilityRowsAttribute
attribute.

In webkit, this attribute is fulfilled by diving on the row's children
and surfacing any TreeItem elements. This CL represents a port of their
implementation.

Additionally, I noticed a confusing spot where the subrole is being
compared in a long line of role comparisons. I moved this around to be
less foot-gunny/confusing and added more attributes for the OutlineRow
subrole that macOS accessibility suggests are necessary (and exist in
the webkit implementation).

Link to webkit impl:
https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperMac.mm#L2836
https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/accessibility/AccessibilityObject.cpp#L1804

Bug: 868480
Test: Use VoiceOver to navigate the table at https://cookiecrook.com/test/aria/tree/ariatree2.html. Note that the table is no longer announced as empty.
Change-Id: Ibb86049efa23e12875aa9aeda541e0145242e3b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062913Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Auto-Submit: Andy Locascio <andy@slack-corp.com>
Cr-Commit-Position: refs/heads/master@{#744740}