PRESUBMIT: Add iOS ARM64 trybots to default set.

BUG=chromium:436831
TBR=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/25319004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7822 4adac7df-926f-26a2-2b94-8c16560cd09d