Add gapless playback support for AAC playback.

media/filters/legacy_frame_processor.cc

Index: media/filters/legacy_frame_processor.cc
diff --git a/media/filters/legacy_frame_processor.cc b/media/filters/legacy_frame_processor.cc
index 71c280183f2d2f74026d89dae8074f23737c5c44..201f2f3f6ab2ef7c14d70d02b4d144d4d83241c5 100644
--- a/media/filters/legacy_frame_processor.cc
+++ b/media/filters/legacy_frame_processor.cc
@@ -213,6 +213,12 @@ void LegacyFrameProcessor::FilterWithAppendWindow(
         buffer->SetDecodeTimestamp(append_window_start);
         buffer->set_duration(frame_end_timestamp - append_window_start);
 
+        // If this isn't the first buffer discarded by the append window, pull
+        // in the buffer immediately preceding this one for preroll.  This
+        // ensures that the partially trimmed buffer can be correctly decoded.
+        if (itr != buffers.begin())

[acolwell GONE FROM CHROMIUM, 2014/05/16 17:01:11]

This looks like it would only work if both buffers are in the same append. I
don't think this behavior should have to depend on that.

[DaleCurtis, 2014/05/16 17:48:59]

Hmm, since buffers outside the append window are lost forever, seemingly the
only other way would be to always keep the last buffer discarded before the
appendWindowStart across calls.  Is that something you're comfortable with?

[acolwell GONE FROM CHROMIUM, 2014/05/16 20:26:05]

If that is what is necessary. Appending 1 byte at a time should result in the
same outcome as appending the whole thing at once. Obviously having a test to
verify this would be nice. :)

[DaleCurtis, 2014/05/22 23:58:57]

Done.

+          buffer->SetPrerollBuffer(*(itr - 1));
+
         // TODO(dalecurtis): This could also be done with |append_window_end|,
         // but is not necessary since splice frames cover the overlap there.
       } else {