[ios] Do not update NTP snapshot after page load

If the user opens the tabswitcher before the page "finishes loading",
the snapshot will be fetched after the NTP has been removed from the
view hierarchy. So the normal insets cannot be factored in, resulting
in a visual glitch.

Before: https://screencast.googleplex.com/cast/NTg1MzY5NDg1NDY5Mjg2NHw3N2I3YWM0OS0xYw
After: https://screencast.googleplex.com/cast/NjY5Mzk0MjMyNTE0OTY5NnxiOTAxNzdjMS0yNQ

Change-Id: I018fec41548cd2bbead4bdaf523df3bc0669d1a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6880752
Reviewed-by: Asami Doi <asamidoi@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1507807}