Added OnEnable/OnDisable on parameters for special cases

ScriptableRenderPipeline/Core/Volume/VolumeComponent.cs

                 .Select(t => (VolumeParameter)t.GetValue(this))
                 .ToList()
                 .AsReadOnly();
+
+            foreach (var parameter in parameters)
+                parameter.OnEnable();
+            if (parameters == null)
+                return;
+
+            foreach (var parameter in parameters)
+                parameter.OnDisable();
         }
 
         public void SetAllOverridesTo(bool state)

ScriptableRenderPipeline/Core/Volume/VolumeParameter.cs

 
         internal abstract void SetValue(VolumeParameter parameter);
 
+        // This is used in case you need to access fields/properties that can't be accessed in the
+        // constructor of a ScriptableObject (VolumeParameter are generally declared and inited in
+        // a VolumeComponent which is a ScriptableObject). This will be called right after the
+        // VolumeComponent object has been constructed, thus allowing access to previously
+        // "forbidden" fields/properties.
+        protected internal virtual void OnEnable()
+        {
+        }
+
+        // Called when the parent VolumeComponent OnDisabled is called
+        protected internal virtual void OnDisable()
+        {
+        }
+
         public static bool IsObjectParameter(Type type)
         {
             if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(ObjectParameter<>))