WebUI: Remove unnecessary usage of paper-button CSS mixins.

Bug: 832173
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I982e24b7234617b111e7adfa3c0f83356722ec8d
Reviewed-on: https://chromium-review.googlesource.com/1013280
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551209}
4 files changed