Guard new protobuf target with enable_protobuf==1.

Fixes a gyp error on iOS.

BUG=4642
TBR=aluebs@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46229004

Cr-Original-Commit-Position: refs/heads/master@{#9180}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e2357149eb566f8999736feac8c4c0ebe99ff2e2
1 file changed