Remove ::part.

Source/core/css/SelectorChecker.cpp

Index: Source/core/css/SelectorChecker.cpp
diff --git a/Source/core/css/SelectorChecker.cpp b/Source/core/css/SelectorChecker.cpp
index 309aa22f54d6b8275e59912c6dd4b8ef816394c9..0e6d251191c6d2bde4ad55f81ceea3da8d7b4d01 100644
--- a/Source/core/css/SelectorChecker.cpp
+++ b/Source/core/css/SelectorChecker.cpp
@@ -74,18 +74,10 @@ static bool matchesCustomPseudoElement(const Element* element, const CSSSelector
     if (!root)
         return false;
 
-    if (selector->pseudoType() != CSSSelector::PseudoPart) {
-        const AtomicString& pseudoId = selector->pseudoType() == CSSSelector::PseudoWebKitCustomElement ? element->shadowPseudoId() : element->pseudo();
-        if (pseudoId != selector->value())
-            return false;
-        if (selector->pseudoType() == CSSSelector::PseudoWebKitCustomElement && root->type() != ShadowRoot::UserAgentShadowRoot)
-            return false;
-        return true;
-    }
-
-    if (element->part() != selector->argument())
+    const AtomicString& pseudoId = selector->pseudoType() == CSSSelector::PseudoWebKitCustomElement ? element->shadowPseudoId() : element->pseudo();
+    if (pseudoId != selector->value())
         return false;
-    if (selector->isMatchUserAgentOnly() && root->type() != ShadowRoot::UserAgentShadowRoot)
+    if (selector->pseudoType() == CSSSelector::PseudoWebKitCustomElement && root->type() != ShadowRoot::UserAgentShadowRoot)
         return false;
     return true;
 }