complete support for Electron webview load-commit event

[mykmelez]

The webview-element branch #60 equates the webview event load-commit, as described in Electron webview docs, with the mozbrowser event mozbrowserlocationchange.

That might be correct, for the top-level frame, anyway, even though WebAPI/BrowserAPI/Common_Subset equates that event with the mozbrowserloadend.

But mozbrowserlocationchange doesn't appear to fire for sub-frame loads, whereas load-commit apparently does. And I haven't yet confirmed that the events are equivalent even for the top-frame load. Also, the implementation doesn't yet set the url and isMainFrame properties of the event object.

So we still need to implement complete support for the load-commit event.