The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4953b70f39f00c5be23b84aade0084c025cc709b

commit 4953b70f39f00c5be23b84aade0084c025cc709b
Author: Rune Lillesveen <futhark@chromium.org>
Date: Mon Jul 23 20:42:28 2018

[Squad] Re-add null check for layout parent.

Because of issue 831568, we may end up trying to generate pseudo
elements for element outside the flat tree if we did a getComputedStyle
for all non-flat-tree ancestors and then triggered style invalidation of
the pseudo element's originating element.

This is not a new issue, but [1] boldly tried to make the if-test for a
layout parent a DCHECK. This CL is effectively reverting that change.

This should ultimately be fixed by fixing 831568 and re-introduce DCHECK
instead of the if-test.

[1] https://crrev.com/86f6eefc70ed2aa46969754e207198bd6917ee33

Bug:  862098 , 831568
Change-Id: Id10fdbe28592e492ef6903b82d2fc47bff930ea4
Reviewed-on: https://chromium-review.googlesource.com/1146729
Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577245}
[modify] https://crrev.com/4953b70f39f00c5be23b84aade0084c025cc709b/third_party/blink/renderer/core/css/style_engine_test.cc
[modify] https://crrev.com/4953b70f39f00c5be23b84aade0084c025cc709b/third_party/blink/renderer/core/dom/element.cc