[SVGDom] Initial <pattern> support

https://www.w3.org/TR/SVG/pservers.html#Patterns

Change-Id: I80455c4ae04cf03526f8e8797f40b0b2d24e043f
Reviewed-on: https://skia-review.googlesource.com/58461
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Robert Phillips <robertphillips@google.com>
8 files changed