Revert of Revert of Clang on Windows: Remove -Wno-unused-variable. (patchset #1 id...

Revert of Revert of Clang on Windows: Remove -Wno-unused-variable. (patchset #1 id:1 of https://codereview.chromium.org/1228093005/)

Reason for revert:
might be better now?

Original issue's description:
> Revert of Clang on Windows: Remove -Wno-unused-variable. (patchset #3 id:40001 of https://codereview.chromium.org/1226573002/)
>
> Reason for revert:
> Broke Clang build on Windows according to http://crbug.com/505319#c30.
>
> Those warnings should be fixed before relanding.
>
> Original issue's description:
> > Clang on Windows: Remove -Wno-unused-variable.
> >
> > Unused variables are now errors. However, since lots of third-party code
> > has unused variables, added a blanket suppression to all third-party
> > code.
> >
> > BUG=505319
> >
> > Committed: https://crrev.com/3371ccbd9d83e1031c5891520e88acbc9c1859a0
> > Cr-Commit-Position: refs/heads/master@{#338242}
>
> TBR=garykac@chromium.org,thakis@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=505319
>
> Committed: https://crrev.com/ff68a3ba6f2dc9b226047b3355c135f73352570e
> Cr-Commit-Position: refs/heads/master@{#338253}

TBR=garykac@chromium.org,mgiuca@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=505319

Review URL: https://codereview.chromium.org/1232543005

Cr-Commit-Position: refs/heads/master@{#338395}