Only handle CE default styles that have rule sets

Some CE default styles might not have RuleSets, this CL adds some
null checks.

Bug: 895512

Change-Id: I02e0f5070c36c72e8d03ed950d96783506f05c5a
Reviewed-on: https://chromium-review.googlesource.com/c/1343585
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609990}

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

@@ -215,8 +215,12 @@ static void MatchCustomElementRules(const Element& element,
           element.GetCustomElementDefinition()) {
     if (definition->HasDefaultStyleSheets()) {
       for (CSSStyleSheet* style : definition->DefaultStyleSheets()) {
-        collector.CollectMatchingRules(MatchRequest(
-            element.GetDocument().GetStyleEngine().RuleSetForSheet(*style)));
+        if (!style)
+          continue;
+        RuleSet* rule_set =
+            element.GetDocument().GetStyleEngine().RuleSetForSheet(*style);
+        if (rule_set)
+          collector.CollectMatchingRules(MatchRequest(rule_set));
       }
     }
   }
@@ -448,7 +452,6 @@ void StyleResolver::MatchAuthorRules(const Element& element,
     MatchAuthorRulesV0(element, collector);
     return;
   }
-
   MatchHostAndCustomElementRules(element, collector);
 
   ScopedStyleResolver* element_scope_resolver = ScopedResolverFor(element);

third_party/blink/renderer/core/css/style_engine.h

@@ -272,8 +272,10 @@ class CORE_EXPORT StyleEngine final
     CollectUserStyleFeaturesTo(features);
     CollectScopedStyleFeaturesTo(features);
     for (CSSStyleSheet* sheet : custom_element_default_style_sheets_) {
-      if (sheet)
-        features.Add(RuleSetForSheet(*sheet)->Features());
+      if (!sheet)
+        continue;
+      if (RuleSet* rule_set = RuleSetForSheet(*sheet))
+        features.Add(rule_set->Features());
     }
   }