Adds support for AVAudioSessionSilenceSecondaryAudioHintNotification on iOS
Adds support for AVAudioSessionSilenceSecondaryAudioHintNotification on iOS

BUG=b/30944297
NOTRY=TRUE

Review-Url: https://codereview.webrtc.org/2366753005
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14398}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f1363fdf578c0ca317609aaff901743cfb89b8e7

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