Auto generate UseCounter::Feature enum from an .in file

Source/build/scripts/templates/UseCounterGenerated.h.tmpl

+{% from 'macros.tmpl' import license %}
+{{license()}}
+
+#ifndef UseCounterGenerated_h
+#define UseCounterGenerated_h
+
+namespace blink {
+
+class UseCounterGenerated {
+public:
+    enum Feature {
+        {% for feature in features %}
+        {{ feature.name }} = {{ feature.value|unique_value(feature.name) }},
+        {% endfor %}
+
+        // This enum value must be last.
+        NumberOfFeatures
+    };
+};
+
+} // namespace blink
+
+#endif // UseCounterGenerated_h