Preloading: Remove unnecessary `explicit` keywords

Remove unnecessary `explicit` keywords from constructors
that take multiple arguments.

Change-Id: I7c795ee826da017bd2195342c6c95c8f3c6dd08c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4617633


Auto-Submit: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1158722}