CSS: Retire flag CSSSelectorsFocusWithin

The :focus-within pseudo-class shipped in Chrome 60.

The runtime flag is no longer needed.

BUG=617371

Change-Id: I12db298d40df458c24987e7928d85aea179e6046
Reviewed-on: https://chromium-review.googlesource.com/662984Reviewed-by: Manuel Rego Casasnovas <rego@igalia.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501599}

third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp

@@ -449,10 +449,6 @@ std::unique_ptr<CSSParserSelector> CSSSelectorParser::ConsumePseudo(
   bool has_arguments = token.GetType() == kFunctionToken;
   selector->UpdatePseudoType(value, *context_, has_arguments, context_->Mode());
 
-  if (!RuntimeEnabledFeatures::CSSSelectorsFocusWithinEnabled() &&
-      selector->GetPseudoType() == CSSSelector::kPseudoFocusWithin)
-    return nullptr;
-
   if (selector->Match() == CSSSelector::kPseudoElement &&
       disallow_pseudo_elements_)
     return nullptr;

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5

@@ -284,10 +284,6 @@
       name: "CSSScrollSnapPoints",
       status: "test",
     },
-    {
-      name: "CSSSelectorsFocusWithin",
-      status: "stable",
-    },
     {
       name: "CSSSnapSize",
       status: "experimental",