[Fixed Position] Change default behavior for old elements

For elements created before this change where useGrid is not defined,
default it to true to ensure consistent display size

Inline constant definitions in unit tests if they are only used once

platform/features/layout/test/elements/ElementProxySpec.js

@@ -24,8 +24,6 @@ define(
     ['../../src/elements/ElementProxy'],
     function (ElementProxy) {
 
-        var GRID_SIZE = [13,21];
-
         describe("A fixed position element proxy", function () {
             var testElement,
                 testElements,
@@ -45,7 +43,7 @@ define(
                     testElement,
                     testElements.indexOf(testElement),
                     testElements,
-                    GRID_SIZE
+                    [13,21]
                 );
             });