Disable size_t truncation warning (C4267) on BoringSSL Win64.

There's a ton of them; OpenSSL is very far from being size_t-clean. Disable
the warning (as in the standalone build's cmake setup) until we get a handle
on it in BoringSSL.

BUG=429039, 167187

Review URL: https://codereview.chromium.org/684093011

Cr-Commit-Position: refs/heads/master@{#302857}
4 files changed