[Fixed Position] Add fill properties

Add 'fill' properties, to allow specifying background
color in Fixed Position view. WTD-881.

platform/features/layout/src/elements/TelemetryProxy.js

@@ -1,16 +1,26 @@
 /*global define*/
 
 define(
-    ['./ElementProxy'],
-    function (ElementProxy) {
+    ['./TextProxy', './AccessorMutator'],
+    function (TextProxy, AccessorMutator) {
         'use strict';
 
         /**
+         * Selection proxy for telemetry elements in a fixed position view.
          *
+         * Note that arguments here are meant to match those expected
+         * by `Array.prototype.map`
+         *
+         * @constructor
+         * @param element the fixed position element, as stored in its
+         *        configuration
+         * @param index the element's index within its array
+         * @param {Array} elements the full array of elements
          */
         function TelemetryProxy(element, index, elements) {
-            var proxy = new ElementProxy(element, index, elements);
+            var proxy = new TextProxy(element, index, elements);
 
+            // Expose the domain object identifier
             proxy.id = element.id;
 
             return proxy;