Reduce memory regression introduced by fixing lifetime of scroll

animations change

https://chromium-review.googlesource.com/c/chromium/src/+/2223998 change
introduced memory regression as a result of adding HeapHashSet data
member to NodeRareData. This change replaces HeapHashSet with
Member<HeapHashSet...>, thus reducing additional memory from 24 to
16 bytes on windows per NodeRareData.

Bug: 1093776
Change-Id: Iaaecd9fc7d2221313f3949a790408a2c88a64b0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2241190Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Olga Gerchikov <gerchiko@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#779060}

third_party/blink/renderer/core/dom/node_rare_data.cc

@@ -44,9 +44,8 @@
 namespace blink {
 
 struct SameSizeAsNodeRareData {
-  Member<void*> willbe_member_[4];
+  Member<void*> willbe_member_[5];
   unsigned bitfields_;
-  HeapHashSet<Member<void*>> scroll_timelines_;
 };
 
 static_assert(sizeof(NodeRareData) == sizeof(SameSizeAsNodeRareData),
@@ -111,10 +110,14 @@ NodeRenderingData& NodeRenderingData::SharedEmptyData() {
 }
 
 void NodeRareData::RegisterScrollTimeline(ScrollTimeline* timeline) {
-  scroll_timelines_.insert(timeline);
+  if (!scroll_timelines_) {
+    scroll_timelines_ =
+        MakeGarbageCollected<HeapHashSet<Member<ScrollTimeline>>>();
+  }
+  scroll_timelines_->insert(timeline);
 }
 void NodeRareData::UnregisterScrollTimeline(ScrollTimeline* timeline) {
-  scroll_timelines_.erase(timeline);
+  scroll_timelines_->erase(timeline);
 }
 
 void NodeRareData::TraceAfterDispatch(blink::Visitor* visitor) const {

third_party/blink/renderer/core/dom/node_rare_data.h

@@ -222,7 +222,7 @@ class GC_PLUGIN_IGNORE("Manual dispatch implemented in NodeData.") NodeRareData
   Member<FlatTreeNodeData> flat_tree_node_data_;
   // Keeps strong scroll timeline pointers linked to this node to ensure
   // the timelines are alive as long as the node is alive.
-  HeapHashSet<Member<ScrollTimeline>> scroll_timelines_;
+  Member<HeapHashSet<Member<ScrollTimeline>>> scroll_timelines_;
 
   DISALLOW_COPY_AND_ASSIGN(NodeRareData);
 };