Simplify snap area rect calculation

Use the kIgnoreScrollOffset flag when calculating a snap area's rect
relative to its snap container, rather than adding the scroll offset
later in the calculation.

Bug: 1016785
Change-Id: Ifbc1c46c5f95e0d8871286be260c2ee219e15e4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902008Reviewed-by: Yi Gu <yigu@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Kaan Alsan <alsan@google.com>
Cr-Commit-Position: refs/heads/master@{#714239}

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

@@ -277,22 +277,11 @@ cc::SnapAreaData SnapCoordinator::CalculateSnapAreaData(
   // tree, as the snap_container is found by walking up the layout tree in
   // FindSnapContainer(). Under this assumption,
   // snap_area.LocalToAncestorRect() returns the snap_area's position relative
-  // to its container's border box. And the |area| below represents the
-  // snap_area rect in respect to the snap_container.
+  // to the snap container's border box, while ignoring scroll offset.
   PhysicalRect area_rect = snap_area.PhysicalBorderBoxRect();
-  area_rect = snap_area.LocalToAncestorRect(area_rect, &snap_container,
-                                            kTraverseDocumentBoundaries);
-  ScrollableArea* scrollable_area =
-      ScrollableArea::GetForScrolling(&snap_container);
-
-  if (scrollable_area) {
-    if (snap_container.IsLayoutView()) {
-      area_rect = snap_container.GetFrameView()->FrameToDocument(area_rect);
-    } else {
-      area_rect.Move(PhysicalOffset::FromFloatPointRound(
-          scrollable_area->ScrollPosition()));
-    }
-  }
+  area_rect = snap_area.LocalToAncestorRect(
+      area_rect, &snap_container,
+      kTraverseDocumentBoundaries | kIgnoreScrollOffset);
 
   LayoutRectOutsets area_margin(
       area_style->ScrollMarginTop(), area_style->ScrollMarginRight(),