DevTools: Allow dynamic module registration only in tests

LayoutTests/http/tests/inspector/layers-test.js

 function initialize_LayerTreeTests()
 {
     // FIXME: remove once out of experimental.
-    runtime.registerModule("layers");
+    InspectorTest.registerModule("layers");
     var extensions = runtime.extensions(WebInspector.Panel).forEach(function(extension) {
         if (extension.module().name() === "layers")
             WebInspector.inspectorView.addPanel(new WebInspector.RuntimeExtensionPanelDescriptor(extension));
     });
     InspectorTest.layerTreeModel = WebInspector.inspectorView.panel("layers")._model;
     InspectorTest.layers3DView = WebInspector.inspectorView.panel("layers")._layers3DView;
 
     InspectorTest.labelForLayer = function(layer)
     {
         var node = layer.nodeForSelfOrAncestor();
@@ skipping 116 matching lines @@
         };
         if (eventType === "mouseout") {
             eventArguments.screenX = 0;
             eventArguments.screenY = 0;
             eventArguments.clientX = 0;
             eventArguments.clientY = 0;
         }
         element.dispatchEvent(new MouseEvent(eventType, eventArguments));
     }
 }