[SVG] Implement currentColor for fill and stroke

The 'color' presentation attribute when set determines the value of the
special color value 'currentColor'. See the color-prop-01-b test for an
example.

Not handled in this CL:
- The color type needs to be changed to be inheritable.
- currentColor should also be usable for stop-color for gradients. See
  <https://www.w3.org/TR/SVG11/color.html#ColorProperty> for a full list
  of attributes that can use currentColor.

Change-Id: Icf81b5313cda688d1b6e20809b9b339f517b9ada
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282638
Commit-Queue: Tyler Denniston <tdenniston@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
6 files changed