GN: Replace Windows suppressions of warning 4267 with config.

This makes the GN configurations easier to read.

BUG=webrtc:5949
NOTRY=True

Review-Url: https://codereview.webrtc.org/2020343003
Cr-Original-Commit-Position: refs/heads/master@{#13006}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8f4419b074b2869e178bb3bd178b8396e4417f3f
8 files changed