[iOS][Omnibox] Replace standard comment with doxygen

Replace standard comment // with doxygen comment ///
This allow Xcode to parse the comment and display them when hinting.

Bug: 1394383
Change-Id: I61e6143d99aa536fe4bd3052e79295c173c64ddc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4061142


Commit-Queue: Christian Xu <christianxu@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Auto-Submit: Christian Xu <christianxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076886}