Implement ::placeholder CSS selector.

third_party/WebKit/Source/core/css/SelectorChecker.cpp

Index: third_party/WebKit/Source/core/css/SelectorChecker.cpp
diff --git a/third_party/WebKit/Source/core/css/SelectorChecker.cpp b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
index 36d3952b711165a0224cb985553c53dbc498832e..3011d8e18ae814783a0a4d0937a1f87df28f2173 100644
--- a/third_party/WebKit/Source/core/css/SelectorChecker.cpp
+++ b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
@@ -1067,9 +1067,15 @@ bool SelectorChecker::checkPseudoElement(const SelectorCheckingContext& context,
       return false;
     }
     case CSSSelector::PseudoWebKitCustomElement: {
-      if (ShadowRoot* root = element.containingShadowRoot())
-        return root->type() == ShadowRootType::UserAgent &&
-               element.shadowPseudoId() == selector.value();
+      if (ShadowRoot* root = element.containingShadowRoot()) {
+        if (root->type() != ShadowRootType::UserAgent)
+          return false;
+        const AtomicString& pseudoName = element.shadowPseudoId();
+        if (pseudoName == selector.value())
+          return true;
+        AtomicString alias = CSSSelector::aliasForShadowPseudoId(pseudoName);
+        return !alias.isEmpty() && alias == selector.value();
+      }
       return false;
     }
     case CSSSelector::PseudoBlinkInternalElement: