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Setting the plugin before attaching print-preview-app to the document
will no longer work in Polymer2. Add an interface for the plugin,
similar to the approach for the NativeLayer, and use this to stub out
checks for plugin compatibility and other plugin functions.

Bug: 738611
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I8428fba35ff5783e18679f99cc02f8d085f82a2e
Reviewed-on: https://chromium-review.googlesource.com/1097514
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567291}