Bug 1491960 [wpt PR 13038] - Snap at ScrollIntoView., a=testonly

Automatic update from web-platform-testsSnap at ScrollIntoView.

According to the spec,
w3c/csswg-drafts#2593 (comment)
scrollIntoView should
1) Always snap to the target element's snap alignment, and
2) All the affected scrollers should also land on a snap position if
one exists.

This patch does part 2). Before a scrollable_area is added to
the smoothScrollSequencer with its new scroll_offset, we will check if
it has a valid snap offset around, and update the final offset
accordingly.

We'll implement part 1) in a separate patch.

Bug: 842317
Cq-Include-Trybots: luci.chromium.try​:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ica22134ed0b1f9d36f5a017c8dd0be54e907dd3d
Reviewed-on: https://chromium-review.googlesource.com/c/1188746
Reviewed-by: David Bokan <bokanchromium.org>
Reviewed-by: Majid Valipour <majidvpchromium.org>
Commit-Queue: Sandra Sun <sunyunjiachromium.org>
Cr-Commit-Position: refs/heads/master{#607019}

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wpt-commits: a0610cf468e6208e74b2cfda77a565ab90fbd74e
wpt-pr: 13038

UltraBlame original commit: e910ba99ad29164d923592600f408c23c2ddcfa2

Author: marco-c