b=933304 apply PannerNode input gain to both channels, even for stereo input r=padenot

--HG--
extra : transplant_source : %BE%A54%E4%8Ezy%AC%FC%BD2%7D%A0h%BB%E0%25%60%94%CA

content/media/webaudio/PannerNode.cpp

@@ -353,8 +353,8 @@ PannerNodeEngine::EqualPowerPanningFunction(const AudioChunk& aInput,
   distanceGain = ComputeDistanceGain();
 
   // Actually compute the left and right gain.
-  gainL = cos(0.5 * M_PI * normalizedAzimuth) * aInput.mVolume;
-  gainR = sin(0.5 * M_PI * normalizedAzimuth) * aInput.mVolume;
+  gainL = cos(0.5 * M_PI * normalizedAzimuth);
+  gainR = sin(0.5 * M_PI * normalizedAzimuth);
 
   // Compute the output.
   if (inputChannels == 1) {
@@ -363,7 +363,7 @@ PannerNodeEngine::EqualPowerPanningFunction(const AudioChunk& aInput,
     GainStereoToStereo(aInput, aOutput, gainL, gainR, azimuth);
   }
 
-  DistanceAndConeGain(aOutput, distanceGain * coneGain);
+  DistanceAndConeGain(aOutput, distanceGain * coneGain * aInput.mVolume);
 }
 
 void