Move the CSS Device Adaptation @viewport rule support behind a runtime flag

Source/core/css/RuleSet.cpp

Index: Source/core/css/RuleSet.cpp
diff --git a/Source/core/css/RuleSet.cpp b/Source/core/css/RuleSet.cpp
index 580a00440f8b6dce1a487579dc5d6b23a7c1fc78..d283692743f9e3779db269bbc1445f8059a02e98 100644
--- a/Source/core/css/RuleSet.cpp
+++ b/Source/core/css/RuleSet.cpp
@@ -33,6 +33,7 @@
 #include <wtf/MemoryInstrumentationHashSet.h>
 #include <wtf/MemoryInstrumentationVector.h>
 #include "HTMLNames.h"
+#include "RuntimeEnabledFeatures.h"
 #include "core/css/CSSFontSelector.h"
 #include "core/css/CSSKeyframesRule.h"
 #include "core/css/CSSSelector.h"
@@ -341,14 +342,12 @@ void RuleSet::addChildRules(const Vector<RefPtr<StyleRuleBase> >& rules, const M
         }
         else if (rule->isHostRule())
             resolver->ensureScopedStyleResolver(scope->shadowHost())->addHostRule(static_cast<StyleRuleHost*>(rule), hasDocumentSecurityOrigin, scope);
-#if ENABLE(CSS_DEVICE_ADAPTATION)
-        else if (rule->isViewportRule() && resolver) {
+        else if (RuntimeEnabledFeatures::cssViewportEnabled() && rule->isViewportRule() && resolver) {
             // @viewport should not be scoped.
             if (!isDocumentScope(scope))
                 continue;
             resolver->viewportStyleResolver()->addViewportRule(static_cast<StyleRuleViewport*>(rule));
         }
-#endif
         else if (rule->isSupportsRule() && static_cast<StyleRuleSupports*>(rule)->conditionIsSupported())
             addChildRules(static_cast<StyleRuleSupports*>(rule)->childRules(), medium, resolver, scope, hasDocumentSecurityOrigin, addRuleFlags);
     }