build: Move CFI Windows build config under OS=="win" block.

Should unbreak CFI bots (update.sh does not support --print-clang-revision).

BUG=464797
TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/1436813005

Cr-Commit-Position: refs/heads/master@{#359388}
1 file changed