Becky Siegel 6de1489147 Refactor gr-button styling
A few changes:
- Utilize mixins within gr-button internally, so less styles have to be
  overridden by elements implementing their own button styles.
- All buttons have a hover background color, which is 12% black
  overlayed on top of its current background color.

Bug: Issue 7894
Change-Id: I4f2879aa0232912267bbf1290a1c800d024099a6
2017-12-01 16:15:19 -08:00
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