style: Tweak background-color and color handling in ignored-colors mode.
style: Tweak background-color and color handling in ignored-colors mode.

We're resetting `color` to the default color when there's a declaration that
applies in order to make stuff like this:

<div style="color: transparent">
  <div style="color: red">
    Red
  </div>
</div>

To not show transparent. But the behavior we want is more like "override with
default color iff there's no other declaration that would set the color from an
user or UA sheet".

This implements that behavior, plus avoids it if we're not inheriting
from transparent, so that stuff like this preserves the behavior from before bug
844349:

<a href="foo">
  <span style="color: red">Should be the red color</span>
</a>

Differential Revision: https://phabricator.services.mozilla.com/D60391

KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo