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According from ARIA 1.1 [1],
default value of aria-level was changed to 2.

We will return 2 as default in aria-level
if not otherwise specified.

[1] https://www.w3.org/WAI/ARIA/wiki/ARIA_1.1_Changes

Bug=688581

Change-Id: Ib03c23f96b74814fe9f52f196c6e0e417059112f
Reviewed-on: https://chromium-review.googlesource.com/670539Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505425}