Remove usage of USE_SYSTEM_PROTOBUF

It's no longer possible to build using the system libprotobuf, so the
macro USE_SYSTEM_PROTOBUF is not set anywhere.  This CL removes all
usages of it.

R=jam@chromium.org

Review-Url: https://codereview.chromium.org/2748203002
Cr-Original-Commit-Position: refs/heads/master@{#462363}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4b7c48089ce5ee9d1a2e4530a82f6bd31e398bbd
1 file changed