Allow variable-length audio packets in AudioSegmentStream

FLAC may have variable-length packets, so allow passing a
duration in samples on incoming frames.

This gets FLAC with, say, 960 samples per frame working
consistently in Firefox and Chrome.

lib/mp4/audio-frame-utils.js

@@ -121,7 +121,7 @@ var generateSampleTable = function(frames) {
     currentFrame = frames[i];
     samples.push({
       size: currentFrame.data.byteLength,
-      duration: 1024 // For AAC audio, all samples contain 1024 samples
+      duration: currentFrame.duration || 1024 // For AAC audio, all samples contain 1024 samples
     });
   }
   return samples;