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Since we can't rely on the modifier flags on ChromeOS, we can't use them
as a way to detect stuck modifier keys. This means that some actions
(including locking the screen) will result in Ctrl and Shift being stuck,
effectively regressing crbug.com/464534.

BUG=337509,484329

Review URL: https://codereview.chromium.org/1145863002

Cr-Commit-Position: refs/heads/master@{#330899}