cros-preview: Add explicit return type

- Add missing explicit return type to functions to align with
  style guide and team norms.

Bug: b:323421684
Test: browser_test --gtest_filter=*PrintPreviewCros*
Change-Id: Ie9b9b193077787049e201a79b2fcf454a343b180
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5334245


Commit-Queue: Ashley Prasad <ashleydp@google.com>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267282}