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Wrap around a dom-if template, which is a thriftier use of resources
since the password dialog should only be shown at the beginning of a
PDF viewing session. More importantly, only a small portion of PDFs
require the dialog.

Rename the element from viewer-password-screen to
viewer-password-dialog.

Fetch password dialog screens before the page is served instead of
using load time data.

Change-Id: I240b8c416d51e423e177548d773eab98e4ca360a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632265Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845210}