Fix assert with distiller code caused by r419635.

The change had a bug because previously WebContentsObserver::DidNavigateMainFrame was listened to which is only called for main frame successful navigations. It was switched to WebContentsObserver::DidFinishNavigation so that it can work with PlzNavigate. However that method is called in more situations, so restrict it to maintain the old behavior.

BUG=650472

Review-Url: https://codereview.chromium.org/2369923003
Cr-Commit-Position: refs/heads/master@{#421233}