[skottie] Initial repeater support

TBR=reed
Bug: skia:8399
Change-Id: Iadaf6b7b363e345d736efd3fd3a5f401963a457b
Reviewed-on: https://skia-review.googlesource.com/c/188631
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
diff --git a/modules/sksg/src/SkSGGroup.cpp b/modules/sksg/src/SkSGGroup.cpp
index df696c4..fff7436 100644
--- a/modules/sksg/src/SkSGGroup.cpp
+++ b/modules/sksg/src/SkSGGroup.cpp
@@ -24,6 +24,13 @@
     }
 }
 
+void Group::clear() {
+    for (const auto& child : fChildren) {
+        this->unobserveInval(child);
+    }
+    fChildren.clear();
+}
+
 void Group::addChild(sk_sp<RenderNode> node) {
     // should we allow duplicates?
     for (const auto& child : fChildren) {