[wip] simplify -webkit-appearance adjust step

[wip] simplify -webkit-appearance adjust step

work in progress; depends on https://codereview.chromium.org/350333003

- simply store a bool on whether backgrounds/borders have changed instead of storing the values
- if you explicitly set the same thing as the ua style, we'll consider it styled (need to rebaseline select-style.html)
- I think this fixes another visited link accessor hole :\

BUG=