clang/win: Don't disable warnings-as-errors for 3rd-party libs.

Not needed: On 32-bit builds, warnings-as-errors are disabled globally. On
64-bit builds, nothing warns, including 3rd-party code.

BUG=82385

Review URL: https://codereview.chromium.org/1011383007

Cr-Commit-Position: refs/heads/master@{#321433}
1 file changed