[ios] Update NewTabPageTabHelper page navigation observation

Update NTPTabHelper to listen to PageLoaded for completed navigation
signals instead of DidStopLoading since it is less reliable than the
former. DidStartNavigation serves to listen for navigations away from
the NTP, so DidStartLoading is also not needed.

-tabDidLoadURL:transitionType:'s implementation in BVC is removed
since it sends a redundant signal to NTPTabHelper for what
DidStartNavigation already does, and for invalid entered URLs,
this is called when no navigation abort signal is received now that
DidStopLoading is not being observed.

Change-Id: If25de2eabc17b3f2ac2e938aac748b7f8084584f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4215921
Reviewed-by: David Jean <djean@chromium.org>
Reviewed-by: Scott Yoder <scottyoder@google.com>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1102876}