clang/win: Fix -Wsign-compare warning after https://codereview.chromium.org/1631903002

Also turn on chromium_code for these targets in gyp.  This makes warnings
for these targets consistent between gyp and gn.

BUG=550983, 581133, 82385
TBR=grt@chromium.org

Review URL: https://codereview.chromium.org/1637793003

Cr-Commit-Position: refs/heads/master@{#371520}
2 files changed