Add build flag to enable bitcode on ios

BUG=webrtc:5085
NOTRY=true

Review-Url: https://codereview.webrtc.org/2478663002
Cr-Original-Commit-Position: refs/heads/master@{#14997}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 592baaf89aaf79245a55bfd3dbf47371b486cd03
2 files changed