[Omnibox][LO] Update lens overlay page classification not reached

Update the NOTREACHED_IN_MIGRATION to NOTREACHED not fatal until, in the lens overlay page classification on iOS.

This can be reached due to an incorrect state detailed in the bug, it
should not be fatal while the feature is still in development.

Bug: 373020297
Change-Id: I998825e2e215eba300263510c3dd51b88b162338
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5933963
Auto-Submit: Christian Xu <christianxu@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1368708}