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Unlike the old implementation, this one leverages Polymer bindings
to hide/show toolbar children, while the document transitions from
loading to loaded.

Fixed: 1100430
Change-Id: I0d54c69c45687295ad309c9215a6b7d38dcbfd05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274861
Commit-Queue: dpapad <dpapad@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784226}