Some modifications to the styling of paper-button within gr-button
overrode a few deliberate styling decisions made in the delete comment
button (namely, color and padding). This change moves those CSS props
inside a styling mixin so that they are properly applied, and overload
the defaults from inside gr-button.
Change-Id: Ifdfc0a3959f4131e1bb86e8b66d70db2fde79077