Do not change video decoder collection unless it is indeed a media stream.

webkit/media/filter_helpers.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "webkit/media/filter_helpers.h"
 
 #include "base/bind.h"
 #include "media/base/filter_collection.h"
 #include "media/base/message_loop_factory.h"
 #include "media/filters/chunk_demuxer_factory.h"
@@ skipping 17 matching lines @@
   filter_collection->AddVideoDecoder(new media::FFmpegVideoDecoder(
       base::Bind(&media::MessageLoopFactory::GetMessageLoop,
                  base::Unretained(message_loop_factory),
                  "VideoDecoderThread")));
 }
 
 bool BuildMediaStreamCollection(const WebKit::WebURL& url,
                                 MediaStreamClient* client,
                                 media::MessageLoopFactory* message_loop_factory,
                                 media::FilterCollection* filter_collection) {
   if (!client)

[Ami GONE FROM CHROMIUM, 2012/03/14 22:00:09]

I thought this was what prevented non-mediastreams from clearing decoders.
In what scenario is client!=NULL but client->GetVideoDecoder()==NULL ?

     return false;
 
+  scoped_refptr<media::VideoDecoder> video_decoder = client->GetVideoDecoder(
+      url, message_loop_factory);
+  if (!video_decoder)
+    return false;
+
   // Remove any "traditional" decoders (e.g. GpuVideoDecoder) from the
   // collection.
   // NOTE: http://crbug.com/110800 is about replacing this ad-hockery with
   // something more designed.
   scoped_refptr<media::VideoDecoder> old_videodecoder;
   do {
     filter_collection->SelectVideoDecoder(&old_videodecoder);
   } while (old_videodecoder);
 
-  scoped_refptr<media::VideoDecoder> video_decoder = client->GetVideoDecoder(
-      url, message_loop_factory);
-  if (!video_decoder)
-    return false;
-
   filter_collection->AddVideoDecoder(video_decoder);
 
   // TODO(vrk/wjia): Setting true for local_source is under the assumption
   // that the MediaStream represents a local webcam. This will need to
   // change in the future when GetVideoDecoder is no longer hardcoded to
   // only return CaptureVideoDecoders.
   scoped_ptr<media::DemuxerFactory> demuxer_factory(
       new media::DummyDemuxerFactory(true, false, true));
   filter_collection->SetDemuxerFactory(demuxer_factory.Pass());
 
@@ skipping 21 matching lines @@
                             media::FilterCollection* filter_collection) {
   scoped_ptr<media::DemuxerFactory> demuxer_factory(
       new media::FFmpegDemuxerFactory(
           data_source, message_loop_factory->GetMessageLoop("PipelineThread")));
   filter_collection->SetDemuxerFactory(demuxer_factory.Pass());
 
   AddDefaultDecodersToCollection(message_loop_factory, filter_collection);
 }
 
 }  // webkit_media