Remove the ENABLE_IFRAME_SEAMLESS compile-time flag.

Source/WebCore/css/SelectorChecker.cpp

Index: Source/WebCore/css/SelectorChecker.cpp
diff --git a/Source/WebCore/css/SelectorChecker.cpp b/Source/WebCore/css/SelectorChecker.cpp
index 314c7db089a72ac1257fee2c67b9a313d7032ba9..34e495019a8d66f1fa959a600ba9bb4b0ec53f36 100644
--- a/Source/WebCore/css/SelectorChecker.cpp
+++ b/Source/WebCore/css/SelectorChecker.cpp
@@ -696,12 +696,10 @@ bool SelectorChecker::checkOne(const SelectorCheckingContext& context, const Sib
                 return false;
             return true;
 #endif
-#if ENABLE(IFRAME_SEAMLESS)
         case CSSSelector::PseudoSeamlessDocument:
             // While a document is rendered in a seamless iframe, the 'seamless-document' pseudoclass applies
             // to all elements of that Document.
             return element->document()->shouldDisplaySeamlesslyWithParent();
-#endif
         case CSSSelector::PseudoInRange:
             if (!element)
                 return false;