String for ChromeVox prompt at login
String for ChromeVox prompt at login

BUG=496365

As an alternative to fixing this bug for M-44, we're adding a voice
label to prompt a ChromeVox user to use TAB to navigate login elements.
This is just the string component of that, to get translation started.

Review URL: https://codereview.chromium.org/1165613005

Cr-Commit-Position: refs/heads/master@{#332727}
(cherry picked from commit 969b033beb375dee99fd01aaab2a063465fc94c4)

Review URL: https://codereview.chromium.org/1145403007

Cr-Commit-Position: d34a0005849006230ae7ffb1a9693419810a06bc@{#196}
Cr-Branched-From: 40cda19e609fd36fbf5fe489055423e611631abc@{#330231}

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