Enable MSVC warning for unused locals.
There is seemingly a bug in the compiler where it occasionally claims a local is
unused when it isn't. This forces a few places to either inline such locals or
mark them ALLOW_UNUSED_LOCAL.
BUG=81439
TEST=none
R=brettw@chromium.org, cpu@chromium.org, jamesr@chromium.org, rvargas@chromium.org, sievers@chromium.org, sky@chromium.org, vitalybuka@chromium.org, wolenetz@chromium.org
Review URL: https://codereview.chromium.org/
731373002
Cr-Commit-Position: refs/heads/master@{#305108}