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In ARIA 1.1, aria-posinset and aria-setsize was added
to "article" role.

This patch will add above properties to article,
and include some changes for "aria-posinset"
in content_browsertests.

Bug: none
Change-Id: I98647cc4d938a0dcf054290f55bbddbdc7eab087
Reviewed-on: https://chromium-review.googlesource.com/810328
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523927}