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Delegate more logic from pdf-viewer into viewer-zoom-toolbar.

Bug: None
Change-Id: I4a80f895ce26825dc491cd7eb4291afdc4b5292f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285360Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786546}