WebUI MD Refresh: Restore button size to 13px, respect font prefs.

Bug: 837190,832173,837137
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I185bf0e3c4e0a4bdb3a15d4288c60c91bed1221b
Reviewed-on: https://chromium-review.googlesource.com/1030881
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554197}

ui/webui/resources/cr_elements/paper_button_style_css.html

@@ -28,7 +28,6 @@
         border-radius: 4px;
         color: var(--text-or-bg-color);
         flex-shrink: 0;
-        font-size: 12px;
         font-weight: 500;
         height: 36px;
         margin: 0;