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 283ffa0a4a28c583d9cdf1aab23bd2553cfabd32..0c0016bb4588024620873a7f903947fe449c89a6 100644
--- a/third_party/WebKit/Source/core/css/SelectorChecker.cpp
+++ b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
@@ -1094,6 +1094,12 @@ bool SelectorChecker::checkPseudoElement(const SelectorCheckingContext& context,
       }
       return false;
     }
+    case CSSSelector::PseudoPlaceholder:
+      if (ShadowRoot* root = element.containingShadowRoot()) {
+        return root->type() == ShadowRootType::UserAgent &&
+               element.shadowPseudoId() == "-webkit-input-placeholder";
+      }
+      return false;
     case CSSSelector::PseudoWebKitCustomElement: {
       if (ShadowRoot* root = element.containingShadowRoot())
         return root->type() == ShadowRootType::UserAgent &&