Update modp_b64 URL.

The old URL just redirects to the new URL, so use the new one.

BUG=chromium:476190

Review URL: https://codereview.chromium.org/1391183004

Cr-Original-Commit-Position: refs/heads/master@{#353141}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: da3cb4b52d3a04ce079546eac4dc96182f58602b