audio_processing/agc: Put entire method set_output_will_be_muted() under lock

Setting the member value output_will_be_muted_ in set_output_will_be_muted() was done before the lock.
This caused a data race.

BUG=4477
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44929004

Cr-Commit-Position: refs/heads/master@{#8877}
2 files changed