Fix various flags in the Win64 GN build

-disable chromium_code for third_party/zlib
- suppress a few size_t -> int conversion warnings
- suppress /wd4121 (structure padding) by default;
  was suppressed in GYP but not GN.

Also disable ffmpeg, which doesn't work in the 64-bit build yet.

R=scottmg@chromium.org
TBR=brettw@chromium.org
BUG=432375, 167187

Review URL: https://codereview.chromium.org/737633002

Cr-Original-Commit-Position: refs/heads/master@{#304542}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: de7d0dc942b1e296a0e9bd667f020ffd486cc32e