When writing tests for #4143 I noticed that the spec has box-sizing: border-box for buttons in general but not for <input type=color>.

http://software.hixie.ch/utilities/js/live-dom-viewer/saved/6501

WebKit/Chromium have border-box content-box, Gecko/EdgeHTML have content-box border-box.

I suggest we change it to border-box so it's consistent with other buttons.