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According to specs http://rawgit.com/w3c/aria/master/core-aam/core-aam.html

aria-pressed {true, mixed} should set STATE_SYSTEM_PRESSED and false should
clear STATE_SYSTEM_PRESSED

This CL depends on https://codereview.chromium.org/767183002/

BUG=171625

Review URL: https://codereview.chromium.org/771583002

Cr-Commit-Position: refs/heads/master@{#308073}