Use 15fps for iPhone 4S

NOTRY=True
BUG=

Review-Url: https://codereview.webrtc.org/2188883002
Cr-Original-Commit-Position: refs/heads/master@{#13564}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d7629105928b5062d604cc1dd19fa12c5f7386ff
6 files changed