Fix the flushing process of DecryptingAudioDecoder.

Fix the flushing process of DecryptingAudioDecoder.

This is the audio part of r277270.

When Decode(EOS) is called on DecryptingAudioDecoder, the decoder should keep
flushing the decryptor with EOS input until NeedMoreData() is returned. Then the
decoder can return the DecodeCB.

BUG=338529
TEST=Fix unittest to cover this case.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277836

[xhwang, 2014-06-16 18:04:58]

This is the audio one. PTAL

Note that in the last CL and this one, DAD/DVD are not returning EOS frames
after the decoder is flushed. This is okay now since they have an internal state
kDecodingFinished. The next time DecoderStream calls Decode(), DAD/DVD will
return EOS frame so that the media pipeline will get notified. This is not
consistent with other decoders and is tricky. I prepared
https://codereview.chromium.org/339653003/ trying to make this whole process
cleaner.

[Sergey Ulanov, 2014-06-16 23:28:44]

lgtm

[xhwang, 2014-06-17 01:53:37]

The CQ bit was checked by xhwang@chromium.org

[commit-bot: I haz the power, 2014-06-17 01:55:20]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/xhwang@chromium.org/332253002/1

[commit-bot: I haz the power, 2014-06-17 11:52:08]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-06-17 11:52:09]

Try jobs failed on following builders:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[xhwang, 2014-06-17 16:40:54]

I added OutputCB(EOS) back after the decoder is flushed.

It turned out that there are cases my original PS doesn't work:

1. DecoderStream has not ready frames.
2. DecoderStream::Read(read_cb)
3. ReadFromDemuxerStream() and get EOS buffer back.
4. Decoder::Decode(EOS, decode_cb)
5. No frames buffered in the decoder. So no OutputCB fired.
6. |decode_cb| fired with kOk.
7. |read_cb_| will not be satisfied.

With my next CL (https://codereview.chromium.org/339653003/), I'll fix
DecoderStream such that I can really drop the OutputCB(EOS) call.

PTAL again. Thanks!

[Sergey Ulanov, 2014-06-17 17:42:47]

lgtm

[xhwang, 2014-06-17 17:48:33]

The CQ bit was checked by xhwang@chromium.org

[commit-bot: I haz the power, 2014-06-17 17:49:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/xhwang@chromium.org/332253002/20001

[commit-bot: I haz the power, 2014-06-17 19:21:24]

Change committed as 277836