[CodeHealth][offline_pages]Add curly braces as per the latest style guide - 1

Add a curly brace when the body is only one line.

Guideline: https://google.github.io/styleguide/cppguide.html#Formatting_Looping_Branching

Bug: 346952289
Change-Id: I55075edf38bf306066e3dee8d430d04e383c5379
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5824119


Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1349516}