Fixed bugs caused by recently added commands.

Page.getResourceTree must return frame id. Also the url should point
to a existing file, since this file will be marked as "main" in
source tree.

WebInspector.onWebSocketConnected must not call
WebInspector._doLoadedDoneWithCapabilities() since it will be called
from Worker.canInspectWorkers response callback. Then we cannot call
WebInspector.showPanel either, since panels are not created yet.
v8NameToInspectorUrl for `evaluate` scripts

Call to `evaluate` creates a new script with no URL.
This commit fixes v8NameToInspectorUrl so that it
returns empty string as URL in this case.