[ios/signin] Convert some DUMP_WILL_BE_CHECK to CHECK

Details in https://crbug.com/40945692#comment15.

Bug: 40945692
Change-Id: Id89a08274e071cf7b6515c42dce2856ef8bf43a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5412799


Auto-Submit: Victor Vianna <victorvianna@google.com>
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281705}