Remove NOTIFICATION_RENDER_VIEW_HOST_CHANGED.
Use WebContentsObserver::RenderViewHostChanged() instead.
BUG=
170921
Test=No behavioral change.
Sanity checked with JS speech, whether WebContentsTracker class gets created and destroyed properly.
Committed:
https://crrev.com/1cf60af0cc6019f03d4cb89c5b3c4788730d3a5e
Cr-Commit-Position: refs/heads/master@{#330213}