[ScrollSnap] Invalidate SnapContainer when applying transform to target

This is a follow-up patch of crrev.com/c/1962567 to support transform
inducing resnapping.

Bug: 1028316
Change-Id: Ie1ae2f088919665c6ddfbf3d8a9dce119d066f8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2008061Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732965}

third_party/blink/renderer/core/layout/layout_box.cc

@@ -491,6 +491,10 @@ void LayoutBox::UpdateScrollSnapMappingAfterStyleChange(
       old_style.ScrollMarginTop() != StyleRef().ScrollMarginTop() ||
       old_style.ScrollMarginRight() != StyleRef().ScrollMarginRight())
     snap_coordinator.SnapAreaDidChange(*this, StyleRef().GetScrollSnapAlign());
+
+  // Transform invalidates the snap area.
+  if (old_style.Transform() != StyleRef().Transform())
+    snap_coordinator.SnapAreaDidChange(*this, StyleRef().GetScrollSnapAlign());
 }
 
 void LayoutBox::AddScrollSnapMapping() {

third_party/blink/web_tests/external/wpt/css/css-scroll-snap/snap-after-relayout/move-current-target-expected.txt

-This is a testharness.js-based test.
-PASS Moving the current snap target should make the scroller resnap to it.
-PASS Changing the layout of other elements should be able to cause resnapping to the target.
-FAIL Transforming the current snap target should make the scroller resnap to it. assert_equals: expected 200 but got 100
-PASS Applying two property changes that do not change the visual offset of the target should not change the scroll offset.
-Harness: the test ran to completion.
-

third_party/blink/web_tests/external/wpt/css/css-scroll-snap/snap-after-relayout/move-current-target.html

@@ -95,8 +95,6 @@ test(t => {
   assert_equals(scroller.scrollTop, 100);
   assert_equals(scroller.scrollLeft, 100);
 
-  // TODO(alsan): Chrome currently doesn't correctly re-snap due to transform.
-  // It seems to be an invalidation issue. http://crbug.com/1028316
   initial_target.style.setProperty("transform", "translate(0,100px)");
   assert_equals(scroller.scrollTop, 200);
   assert_equals(scroller.scrollLeft, 100);