[sksg] Refactor inval registration

... to avoid having too many Node friends.

TBR=
Change-Id: I8f8ff570d94ea48017935066a3d51cd8265ec120
Reviewed-on: https://skia-review.googlesource.com/97980
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/experimental/sksg/geometry/SkSGGeometryTransform.cpp b/experimental/sksg/geometry/SkSGGeometryTransform.cpp
index 14c37d9..91367d4 100644
--- a/experimental/sksg/geometry/SkSGGeometryTransform.cpp
+++ b/experimental/sksg/geometry/SkSGGeometryTransform.cpp
@@ -14,13 +14,13 @@
 GeometryTransform::GeometryTransform(sk_sp<GeometryNode> child, sk_sp<Matrix> matrix)
     : fChild(std::move(child))
     , fMatrix(std::move(matrix)) {
-    fChild->addInvalReceiver(this);
-    fMatrix->addInvalReceiver(this);
+    this->observeInval(fChild);
+    this->observeInval(fMatrix);
 }
 
 GeometryTransform::~GeometryTransform() {
-    fChild->removeInvalReceiver(this);
-    fMatrix->removeInvalReceiver(this);
+    this->unobserveInval(fChild);
+    this->unobserveInval(fMatrix);
 }
 
 SkRect GeometryTransform::onRevalidate(InvalidationController* ic, const SkMatrix& ctm) {