Revert of Remove AudioBuffer::set_duration(), instead base on frames.

media/base/audio_discard_helper.cc

Index: media/base/audio_discard_helper.cc
diff --git a/media/base/audio_discard_helper.cc b/media/base/audio_discard_helper.cc
index d868382ef42c6a639e832484bc4830ac0b4f6254..3088130b00c48ca9f3c7966a4c45eac53e4ca87c 100644
--- a/media/base/audio_discard_helper.cc
+++ b/media/base/audio_discard_helper.cc
@@ -103,8 +103,10 @@
     DCHECK(encoded_buffer->discard_padding() == base::TimeDelta());
   }
 
-  // Assign timestamp to the buffer.
+  // Assign timestamp and duration to the buffer.
   decoded_buffer->set_timestamp(timestamp_helper_.GetTimestamp());
+  decoded_buffer->set_duration(
+      timestamp_helper_.GetFrameDuration(decoded_buffer->frame_count()));
   timestamp_helper_.AddFrames(decoded_buffer->frame_count());
   return true;
 }