Avoid dynamic specificity calculation for ::slotted()

I want to try and remove the dynamic specificity calculation
altogether. After this, it's only used by :host and :host-context.

Also, we currently return kClassLikeSpecificity for pseudo-elements
(for some reason), so use this for ::slotted as well. Even though
it's not observable for authors, it makes things somewhat consistent
internally at least.

Test: external/wpt/css/css-scoping/slotted-specificity.html
Change-Id: Ie64144dfca7439cb02ee839d33c53a409e3cc523
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505751Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821882}

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

@@ -146,8 +146,16 @@ inline unsigned CSSSelector::SpecificityForOneSelector() const {
           break;
       }
       return kClassLikeSpecificity;
-    case kClass:
     case kPseudoElement:
+      switch (GetPseudoType()) {
+        case kPseudoSlotted:
+          DCHECK(SelectorList()->HasOneSelector());
+          return kClassLikeSpecificity + SelectorList()->First()->Specificity();
+        default:
+          break;
+      }
+      return kClassLikeSpecificity;
+    case kClass:
     case kAttributeExact:
     case kAttributeSet:
     case kAttributeList:

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

@@ -123,6 +123,11 @@ TEST(CSSSelector, Specificity_Where) {
       Specificity("*"));
 }
 
+TEST(CSSSelector, Specificity_Slotted) {
+  EXPECT_EQ(Specificity("::slotted(.a)"), Specificity(".a::first-line"));
+  EXPECT_EQ(Specificity("::slotted(*)"), Specificity("::first-line"));
+}
+
 TEST(CSSSelector, HasLinkOrVisited) {
   EXPECT_FALSE(HasLinkOrVisited("tag"));
   EXPECT_FALSE(HasLinkOrVisited("visited"));

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

@@ -1249,9 +1249,6 @@ bool SelectorChecker::CheckPseudoElement(const SelectorCheckingContext& context,
       MatchResult sub_result;
       if (MatchSelector(sub_context, sub_result) != kSelectorMatches)
         return false;
-      result.specificity += sub_context.selector->Specificity() +
-                            sub_result.specificity +
-                            CSSSelector::kTagSpecificity;
       return true;
     }
     case CSSSelector::kPseudoContent: