Avoid touching the geolocation object until needed

IE9 leaks when `navigator.geolocation` is accessed (see #461).  The goal of this change is to avoid that leak in builds that include the Geolocate control but do not use it.

lib/OpenLayers/Control/Geolocate.js

@@ -45,8 +45,15 @@ OpenLayers.Control.Geolocate = OpenLayers.Class(OpenLayers.Control, {
     /**
      * Property: geolocation
      * {Object} The geolocation engine, as a property to be possibly mocked.
+     * This is set lazily to avoid a memory leak in IE9.
      */
-    geolocation: navigator.geolocation,
+    geolocation: null,
+
+    /**
+     * Property: available
+     * {Boolean} The navigator.geolocation object is available.
+     */
+    available: ('geolocation' in navigator),
 
     /**
      * APIProperty: bind
@@ -90,6 +97,10 @@ OpenLayers.Control.Geolocate = OpenLayers.Class(OpenLayers.Control, {
      * {Boolean} The control was effectively activated.
      */
     activate: function () {
+        if (this.available && !this.geolocation) {
+            // set lazily to avoid IE9 memory leak
+            this.geolocation = navigator.geolocation;
+        }
         if (!this.geolocation) {
             this.events.triggerEvent("locationuncapable");
             return false;