[SVGDom] Opacity optimization

Apply opacity as fill/stroke paint alpha instead of saveLayer, when
possible.

R=robertphillips@google.com,stephana@google.com,reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2353503005

Committed: https://skia.googlesource.com/skia/+/3dbb7b9f196d793fbd16243157ee67638891f5dc
Review-Url: https://codereview.chromium.org/2353503005
diff --git a/experimental/svg/model/SkSVGNode.h b/experimental/svg/model/SkSVGNode.h
index dcdb589..2a7c836 100644
--- a/experimental/svg/model/SkSVGNode.h
+++ b/experimental/svg/model/SkSVGNode.h
@@ -69,6 +69,8 @@
 
     virtual void onSetAttribute(SkSVGAttribute, const SkSVGValue&);
 
+    virtual bool hasChildren() const { return false; }
+
 private:
     SkSVGTag                    fTag;