Missing null check for GetComputedStyle().

Made chrome://tracing crash on closing <dialog> element with
FlatTreeStyleRecalc enabled.

Bug: 972752
Change-Id: Ie152f10d2972a873fb7f1cc00cff745719d90faf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1905710Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713809}

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

@@ -1314,9 +1314,11 @@ void Node::MarkAncestorsWithChildNeedsStyleRecalc() {
       ContainerNode* style_parent = ancestor;
       while (style_parent && !style_parent->CanParticipateInFlatTree())
         style_parent = style_parent->GetStyleRecalcParent();
-      if (style_parent &&
-          style_parent->GetComputedStyle()->IsEnsuredOutsideFlatTree()) {
-        return;
+      if (style_parent) {
+        if (const auto* parent_style = style_parent->GetComputedStyle()) {
+          if (parent_style->IsEnsuredOutsideFlatTree())
+            return;
+        }
       }
     }
   }

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

@@ -503,4 +503,28 @@ TEST_F(NodeTest, UpdateChildDirtyAfterSlotRemoval) {
   EXPECT_TRUE(GetDocument().GetStyleEngine().NeedsStyleRecalc());
 }
 
+TEST_F(NodeTest, UpdateChildDirtyAfterSlottingDirtyNode) {
+  ScopedFlatTreeStyleRecalcForTest scope(true);
+
+  SetBodyContent("<div id=host><span></span></div>");
+
+  auto* host = GetDocument().getElementById("host");
+  auto* span = To<Element>(host->firstChild());
+
+  // Make sure the span is style dirty.
+  span->setAttribute("style", "color:green");
+
+  ShadowRoot& shadow_root =
+      host->AttachShadowRootInternal(ShadowRootType::kOpen);
+  shadow_root.SetInnerHTMLFromString("<div><slot></slot></div>");
+
+  GetDocument().GetSlotAssignmentEngine().RecalcSlotAssignments();
+
+  // Make sure shadow tree div and slot are marked with ChildNeedsStyleRecalc
+  // when the dirty span is slotted in.
+  EXPECT_TRUE(shadow_root.firstChild()->ChildNeedsStyleRecalc());
+  EXPECT_TRUE(shadow_root.firstChild()->firstChild()->ChildNeedsStyleRecalc());
+  EXPECT_TRUE(span->NeedsStyleRecalc());
+}
+
 }  // namespace blink