Suppress modifier key auto-repeat on ChromeOS.
Suppress modifier key auto-repeat on ChromeOS.

ChromeOS hosts synthesize keyup events between auto-repeated keydown
events, which is the wrong thing to do in the case of modifier keys.

(See https://codereview.chromium.org/1161173003/ for the equivalent CL on Linux.)

BUG=496431

Review URL: https://codereview.chromium.org/1163073004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#332753}

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