iOS: Set enable_protobuf=1 by default.
iOS: Set enable_protobuf=1 by default.

BUG=webrtc:5235
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1462873003 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10724}

TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc