set G-Y-M layers' isBaseLayer() to true

git-svn-id: http://svn.openlayers.org/trunk/openlayers@623 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/Google.js

@@ -37,7 +37,14 @@ OpenLayers.Layer.Google.prototype = Object.extend( new OpenLayers.Layer(), {
         // once our layer has been added to the map, we can create the vemap
         this.map.events.register("addlayer", this, this.loadGMap);
     },
-
+    
+    /** Google layer is always a base class.
+     * @type Boolean
+     */
+    isBaseLayer: function() {
+        return true;
+    },
+    
     /** 
      * @param {OpenLayers.Bounds} bounds
      * @param {int} zoomChanged

lib/OpenLayers/Layer/VirtualEarth.js

@@ -36,6 +36,13 @@ OpenLayers.Layer.VirtualEarth.prototype =
         this.map.events.register("addlayer", this, this.loadVEMap);
     },
 
+    /** Virtual Earth layer is always a base class. 
+     * @type Boolean
+     */
+    isBaseLayer: function() {
+        return true;
+    },
+
     /** 
      * @param {OpenLayers.Bounds} bounds
      * @param {int} zoomChanged

lib/OpenLayers/Layer/Yahoo.js

@@ -37,6 +37,13 @@ OpenLayers.Layer.Yahoo.prototype = Object.extend( new OpenLayers.Layer(), {
         this.map.events.register("addlayer", this, this.loadYMap);
     },
 
+    /** Yahoo layer is always a base class.
+     * @type Boolean
+     */
+    isBaseLayer: function() {
+        return true;
+    },
+    
     /** 
      * @param {OpenLayers.Bounds} bounds
      * @param {int} zoomChanged