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Aside from adding the sanity check, this also hooks up loadTimeData in
pdf_viewer.js, and demonstrates how to plumb a feature flag from the
browser into pdf_viewer.js.

BUG=61248

Change-Id: I48850afaac17854af785d04a8ca93078ebd8532f
Reviewed-on: https://chromium-review.googlesource.com/c/1339244Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608890}