Make sure style is up-to-date for scrollbar pseudo style.

EnsureStyleResolver used to do this partially, which made us not crash
before [1] landed.

[1] https://crrev.com/0e37e8a49d3cfb0f81fa8a9e8ec147c50914e926

Bug: 1109639, 1104634
Change-Id: Ia4d8246b954251d0d3e566cf29081dd8157a5fcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336081Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794693}

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

@@ -130,11 +130,18 @@ void CustomScrollbar::SetPressedPart(ScrollbarPart part,
 scoped_refptr<const ComputedStyle>
 CustomScrollbar::GetScrollbarPseudoElementStyle(ScrollbarPart part_type,
                                                 PseudoId pseudo_id) {
-  if (!StyleSource()->GetLayoutObject())
+  Element* element = StyleSource();
+  DCHECK(element);
+  Document& document = element->GetDocument();
+  // This method may be called as part of style recalc. Don't re-enter into
+  // updating style for those cases.
+  if (!document.InStyleRecalc())
+    document.UpdateStyleAndLayoutTreeForNode(element);
+  if (!element->GetLayoutObject())
     return nullptr;
-  const ComputedStyle* source_style = StyleSource()->GetLayoutObject()->Style();
+  const ComputedStyle* source_style = element->GetLayoutObject()->Style();
   scoped_refptr<const ComputedStyle> part_style =
-      StyleSource()->StyleForPseudoElement(
+      element->StyleForPseudoElement(
           PseudoElementStyleRequest(pseudo_id, this, part_type), source_style);
   if (!part_style)
     return nullptr;