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This avoids creating a bunch of <option> elements if the timezone
select isn't shown.

Follow-up from this review:
https://chromium-review.googlesource.com/c/chromium/src/+/1536544

Bug: 927732
Test: browser_tests
Change-Id: I330256a57ebd0082b7755af1441c89d1cb83623d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539268Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644803}