[SVGDom] ClipPath support

  * clip-path attribute handling
  * clipPath container element
  * asPath() SkSVGNode virtual for capturing subtree geometry

R=robertphillips@google.com,stephana@google.com

Change-Id: I9597534fe3047b631da6309eafac055dff5696e9
Reviewed-on: https://skia-review.googlesource.com/5650
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 29acd54d..1ee4d60 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -930,6 +930,7 @@
       "experimental/svg/model/SkSVGAttribute.cpp",
       "experimental/svg/model/SkSVGAttributeParser.cpp",
       "experimental/svg/model/SkSVGCircle.cpp",
+      "experimental/svg/model/SkSVGClipPath.cpp",
       "experimental/svg/model/SkSVGContainer.cpp",
       "experimental/svg/model/SkSVGDOM.cpp",
       "experimental/svg/model/SkSVGEllipse.cpp",