[ScrollSnap] Fix initial value of any_snap_container_data_needs_update_

Before the variable was introduced in crrev.com/c/2005822, we updated
all PaintLayerScrollableAreas' SnapContainerData at the end of the first
Document::UpdateStyleAndLayout. Because this variable is now initialized
to false, the update process will be skipped.

Bug: None
Change-Id: I5e08bf31899f6fc161b99673c7bab2e7b7d3c578
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2044813Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740413}

third_party/blink/renderer/core/page/scrolling/snap_coordinator.h

@@ -77,7 +77,7 @@ class CORE_EXPORT SnapCoordinator final
   friend class SnapCoordinatorTest;
 
   HashSet<LayoutBox*> snap_containers_;
-  bool any_snap_container_data_needs_update_ = false;
+  bool any_snap_container_data_needs_update_ = true;
 
   // Used for reporting to UMA when snapping on the initial layout affects the
   // initial scroll position.