Review URL: 5a76cd676a
This implements mouse events on WebKit-Mac. This uses a similar
approach to the GTK port, creating a NativeWebMouseEvent without
a backing native event and dispatching it to WebPageProxy.
WebKit WebDriver implementation takes a different approach where they
employ `[nswindow sendEvent:]` method. Unfortunately for us, it's
quite challenging to make this method work for the headless mode
on OSX.
For future reference: chromium has a nice
"event_generator_delegate_mac.mm" that dispatches events right into
`NSResponder`. This approach didn't work as well for us here.
