gtest.py: Use BUILDFLAG for OS checking

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

Bug: 12340436
Test: No functionality change
Change-Id: I27f285786b41eef9035185fd0db748ec83ffb284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3408188


Auto-Submit: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962540}