[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/SkSGTrimEffect.cpp b/experimental/sksg/geometry/SkSGTrimEffect.cpp
index 4ee85c8..f311822 100644
--- a/experimental/sksg/geometry/SkSGTrimEffect.cpp
+++ b/experimental/sksg/geometry/SkSGTrimEffect.cpp
@@ -15,11 +15,11 @@
 
 TrimEffect::TrimEffect(sk_sp<GeometryNode> child)
     : fChild(std::move(child)) {
-    fChild->addInvalReceiver(this);
+    this->observeInval(fChild);
 }
 
 TrimEffect::~TrimEffect() {
-    fChild->removeInvalReceiver(this);
+    this->unobserveInval(fChild);
 }
 
 // TODO