Revert of Clang on Windows: Remove -Wno-unused-variable.

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}

[Matt Giuca, 2015-07-10 07:52:20]

Created Revert of Clang on Windows: Remove -Wno-unused-variable.

[commit-bot: I haz the power, 2015-07-10 07:52:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1228093005/1

[commit-bot: I haz the power, 2015-07-10 07:53:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-10 07:54:09]

Patchset 1 (id:??) landed as
https://crrev.com/ff68a3ba6f2dc9b226047b3355c135f73352570e
Cr-Commit-Position: refs/heads/master@{#338253}

[Nico, 2015-07-10 23:27:42]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1232543005/ by thakis@chromium.org.

The reason for reverting is: might be better now?.