[SVGDom] Factor out common gradient logic

In preparation of radial gradient support, move common logic into an
abstract base class (SkSVGGradient).

Change-Id: Ie5361048ca8fddd9070c573c8daef0d0f57dc95e
Reviewed-on: https://skia-review.googlesource.com/57108
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 29fc020..96ef7d6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1172,6 +1172,7 @@
       "experimental/svg/model/SkSVGContainer.cpp",
       "experimental/svg/model/SkSVGDOM.cpp",
       "experimental/svg/model/SkSVGEllipse.cpp",
+      "experimental/svg/model/SkSVGGradient.cpp",
       "experimental/svg/model/SkSVGLine.cpp",
       "experimental/svg/model/SkSVGLinearGradient.cpp",
       "experimental/svg/model/SkSVGNode.cpp",