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This is a cleanup use an HTML attribute to position buttons when
Print Preview is hosting the plugin. This allows some simplification
compared to the previous implementation of using Polymer properties.

Change-Id: Icd73f2fa8969dc76579e477a7ad967f2b3ceeaaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880151
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709364}