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Fixed issue by only using the AXTree::GetPosInSet/AXTree::GetSetSize heuristics if they provide a nonzero answer. Otherwise, fall back to raw attributes.
This also exposes pos in set/set size if they are set without doing a role check as some renderers like ARC++ have no such restrictions.

TBR=dmazzoni@chromium.org
Original change description:
This moves ChromeVox to use AXNode::GetSetSize and AXNode::GetPosInSet through their automation bindings.

One issue is fixed:

In AXTree:PopulateOrderedSetItems, an assumption is made that the first child node of the ordered set node, should determine the hierarchical level of the current layer of items.

Unfortunately, for trees like:
tree
  static text
  tree item
  ...

this hueristic fails. See output_test.extjs::NestedLists.

Test: browser_tests --gtest_filter=ChromeVoxOutput*.*
Change-Id: I54911673215e245808a88fab07275a83c2d7109d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536470Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#645669}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560015
Cr-Commit-Position: refs/heads/master@{#649245}