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..a29b23c31bced68efcb50dc8f4d70953bf150076 100644
--- a/third_party/WebKit/Source/core/css/ElementRuleCollector.cpp
+++ b/third_party/WebKit/Source/core/css/ElementRuleCollector.cpp
@@ -196,6 +196,8 @@ void ElementRuleCollector::collectMatchingRules(
     collectMatchingRulesForList(
         matchRequest.ruleSet->shadowPseudoElementRules(pseudoId), cascadeOrder,
         matchRequest);
+    collectMatchingRulesForList(matchRequest.ruleSet->placeholderPseudoRules(),
+                                cascadeOrder, matchRequest);

[rune, 2016/11/21 08:52:33]

This means we'll try to match ::placeholder rules for all custom pseudo
elements, right? Should be guarded by if-test for placeholder pseudo id?

[tkent, 2016/11/21 09:23:39]

Oh, that's right.  Fixed.

   }
 
   if (element.isVTTElement())