Mark mini_installer as chromium_code in gyp builds.

This matches the gn build. Also fix a warning that only fires in
chromium_code mode -- which means that this CL also fixes the
clang/win/gn build of mini_installer.

BUG=82385
R=gab@chromium.org

Review URL: https://codereview.chromium.org/1580673004 .

Cr-Commit-Position: refs/heads/master@{#368983}
2 files changed