Kind of a big change, to connect several new bits into something useful:
* ID tracking & lookup
* new asPaint() node virtual to support shader (and in the future filter) based paint servers
* <defs>, <linearGradient> and <stop> element support
* 'href', 'offset', 'stop-color', 'stop-opacity' attribute support
* IRI/FuncIRI and rgb(...) parsing
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2327233003
Review-Url: https://codereview.chromium.org/2327233003
