fix($route): make asynchronous tasks count as pending requests

Protractor users were having a problem where if they had asynchonous code in a
`route.resolve` or `route.resolveRedirectTo` variable, Protractor was not
waiting for that code to complete before continuing. See
https://github.com/angular/protractor/issues/789#issuecomment-190983200 for
details.

This commit fixes it by ensuring that `$browser#outstandingRequestCount` is
properly increased/decreased while `$route` (asynchronously) processes a route.

Also, enhanced `ngMock` to wait for pending requests, before calling callbacks
from `$browser.notifyWhenNoOutstandingRequests()`.

Related to angular/protractor#789.

Closes #14159