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
4d363ae305/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-Commit-Position: refs/heads/master@{#9816}

webrtc/modules/audio_coding/neteq/background_noise.cc

@@ -246,8 +246,7 @@ void BackgroundNoise::SaveParameters(size_t channel,
   if (norm_shift & 0x1) {
     norm_shift -= 1;  // Even number of shifts required.
   }
-  assert(norm_shift >= 0);  // Should always be positive.
-  residual_energy = residual_energy << norm_shift;
+  residual_energy = WEBRTC_SPL_SHIFT_W32(residual_energy, norm_shift);
 
   // Calculate scale and shift factor.
   parameters.scale = static_cast<int16_t>(WebRtcSpl_SqrtFloor(residual_energy));