Add a public API to enable/disable blink runtime features by name

Source/core/scripts/templates/RuntimeEnabledFeatures.h.tmpl

 {% from "macros.tmpl" import wrap_with_condition, license -%}
 {{ license() }}
 
 #ifndef RuntimeEnabledFeatures_h
 #define RuntimeEnabledFeatures_h
 
+#include "wtf/text/WTFString.h"
+
 namespace WebCore {
 
 // A class that stores static enablers for all experimental features.
 
 class RuntimeEnabledFeatures {
 public:
 
+    static bool setFeatureEnabled(const String& feature, bool enable);
 {%- for feature_set in feature_sets %}
     static void set{{feature_set|capitalize}}FeaturesEnabled(bool);
 {%- endfor %}
 
 {% for feature in features %}
 {%- if feature.custom %}
     static bool {{feature.first_lowered_name}}Enabled();
 {%- else %}
 {%- if feature.condition %}
 #if ENABLE({{feature.condition}})
@@ skipping 15 matching lines @@
 {%- call wrap_with_condition(feature.condition) %}
     static bool is{{feature.name}}Enabled;
 {%- endcall %}
 {%- endfor %}
 };
 
 } // namespace WebCore
 
 #endif // RuntimeEnabledFeatures_h