[ios] Fix NTP flicker when asynchronously loading MVTs

Change-Id: I5cf9643c3aa1e5ba8d95a353c90760c843fc36e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402470


Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Auto-Submit: Scott Yoder <scottyoder@google.com>
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279990}