NetEq: Allow negative shift in BackgroundNoise::SaveParameters
NetEq: Allow negative shift in BackgroundNoise::SaveParameters

This change allows a shift factor to be negative.  This is the way it
was in the old (NetEq3) code; see
https://chromium.googlesource.com/external/webrtc/+/4d363ae305fa975e7f3ad2a5ca5dbe5e3f101f82/webrtc/modules/audio_coding/neteq/bgn_update.c,
lines 183-188.

Some input signals can lead to negative shifts, and would then trigger
an assert. The assert is now removed.

BUG=webrtc:4840
R=minyue@webrtc.org

Review URL: https://codereview.webrtc.org/1315833003 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9816}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3c4ef29140fb1a5e488899e1905a62277f0d9858

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