Update mocks tabs.onCreated and getURL

[jcblw]

Description

To support the chrome apis for mujo-code/chrome-extension#270 we are going to need a few additional mocks.

• tab.onCreated - there are a few on* listener interfaces at the bottom of the tabs file. We can just copy those over.
• runtime.getURL

This should clear up any mock failures in the background pages of Mujō.

[jcblw]

@Temmyogunbo here is the issue let me know if you need any help.