Make modp_b64 and base::base64 compile on Win64

BUG=166496

Review URL: https://chromiumcodereview.appspot.com/11688006

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c525312fcba05ce1b6de26586e83f173f90a89c4
3 files changed