[svg] Refactor <pattern> and gradient stop elements to new parsing

Change-Id: I1fb84760f7a657aefeb7e0af3758766daac2d0f9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/367882
Reviewed-by: Florin Malita <fmalita@chromium.org>
7 files changed