PseudoElement is always created before style resolution.

This changed in [1]. However, there are pseudo elements which do not
have corresponding PseudoElements. Change DCHECK documentation
accordingly.

[1] https://crrev.com/90351b8f23ad3c9195d54e741747a85d95755987

Change-Id: Ib88155e8a846d790367704207f7bc796863bce77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1640638
Auto-Submit: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665574}

third_party/blink/renderer/core/css/resolver/style_resolver.cc

@@ -1132,9 +1132,10 @@ bool StyleResolver::ApplyAnimatedStandardProperties(
   Element* element = state.GetElement();
   DCHECK(element);
 
-  // The animating element may be this element, or its pseudo element. It is
-  // null when calculating the style for a potential pseudo element that has
-  // yet to be created.
+  // The animating element may be this element, the pseudo element we are
+  // resolving style for, or null if we are resolving style for a pseudo
+  // element which is not represented by a PseudoElement like scrollbar pseudo
+  // elements.
   DCHECK(animating_element == element || !animating_element ||
          animating_element->ParentOrShadowHostElement() == element);