chrome/browser/download: Use BUILDFLAG for OS checking

Use BUILDFLAG(IS_XXX) instead of defined(OS_XXX).

Generated by `os_buildflag_migration.py` (https://crrev.com/c/3311983).

R=thakis@chromium.org

Bug: 12340436
Test: No functionality change
Change-Id: Ic39fd5f1bb4bb09af9b853b2795ffb01dd81589c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3374250


Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958213}