Implement ::placeholder CSS selector.

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

Index: third_party/WebKit/Source/core/css/ElementRuleCollector.cpp
diff --git a/third_party/WebKit/Source/core/css/ElementRuleCollector.cpp b/third_party/WebKit/Source/core/css/ElementRuleCollector.cpp
index c77c0da1d62db403fadcb59e1dff9bc28c9c21cd..142483563ffddea3663cf3b3969d027418665f8e 100644
--- a/third_party/WebKit/Source/core/css/ElementRuleCollector.cpp
+++ b/third_party/WebKit/Source/core/css/ElementRuleCollector.cpp
@@ -196,6 +196,12 @@ void ElementRuleCollector::collectMatchingRules(
     collectMatchingRulesForList(
         matchRequest.ruleSet->shadowPseudoElementRules(pseudoId), cascadeOrder,
         matchRequest);
+    AtomicString alias = CSSSelector::aliasForShadowPseudoId(pseudoId);
+    if (!alias.isEmpty()) {
+      collectMatchingRulesForList(
+          matchRequest.ruleSet->shadowPseudoElementRules(alias), cascadeOrder,
+          matchRequest);
+    }
   }
 
   if (element.isVTTElement())