Guard new protobuf target with enable_protobuf==1.
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9180}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e2357149eb566f8999736feac8c4c0ebe99ff2e2

krieger-od/webrtc,sippet/webrtc,jchavanton/webrtc,sippet/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,krieger-od/webrtc,sippet/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,krieger-od/webrtc,jchavanton/webrtc,krieger-od/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,krieger-od/webrtc,sippet/webrtc,Alkalyne/webrtctrunk,krieger-od/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk