[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/1146729Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577245}

third_party/blink/renderer/core/css/style_engine_test.cc

@@ -1455,4 +1455,21 @@ TEST_F(StyleEngineTest, ShadowRootStyleRecalcCrash) {
   GetDocument().View()->UpdateAllLifecyclePhases();
 }
 
+TEST_F(StyleEngineTest, GetComputedStyleOutsideFlatTreeCrash) {
+  GetDocument().body()->SetInnerHTMLFromString(R"HTML(
+    <style>
+      body, div { display: contents }
+      div::before { display: contents; content: "" }
+    </style>
+    <div id=inner></div>
+  )HTML");
+
+  GetDocument().documentElement()->CreateV0ShadowRootForTesting();
+  GetDocument().View()->UpdateAllLifecyclePhases();
+  GetDocument().body()->EnsureComputedStyle();
+  GetDocument().getElementById("inner")->SetInlineStyleProperty(
+      CSSPropertyColor, "blue");
+  GetDocument().View()->UpdateAllLifecyclePhases();
+}
+
 }  // namespace blink

third_party/blink/renderer/core/dom/element.cc

@@ -4143,9 +4143,14 @@ scoped_refptr<ComputedStyle> Element::StyleForPseudoElement(
   if (is_before_or_after) {
     const ComputedStyle* layout_parent_style = style;
     if (style->Display() == EDisplay::kContents) {
-      Node* layout_parent = LayoutTreeBuilderTraversal::LayoutParent(*this);
-      DCHECK(layout_parent);
-      layout_parent_style = layout_parent->GetComputedStyle();
+      // TODO(futhark@chromium.org): Calling getComputedStyle for elements
+      // outside the flat tree should return empty styles, but currently we do
+      // not. See issue https://crbug.com/831568. We can replace the if-test
+      // with DCHECK(layout_parent) when that issue is fixed.
+      if (Node* layout_parent =
+              LayoutTreeBuilderTraversal::LayoutParent(*this)) {
+        layout_parent_style = layout_parent->GetComputedStyle();
+      }
     }
     return GetDocument().EnsureStyleResolver().PseudoStyleForElement(
         this, request, style, layout_parent_style);