Implement opus seek preroll for MediaSourceExtensions.

media/filters/source_buffer_stream.cc

Index: media/filters/source_buffer_stream.cc
diff --git a/media/filters/source_buffer_stream.cc b/media/filters/source_buffer_stream.cc
index 608253b2d87bc5fa38873fbc36a6a6b26f9d7203..a8b929add41bc8545928010358de8304589de3a3 100644
--- a/media/filters/source_buffer_stream.cc
+++ b/media/filters/source_buffer_stream.cc
@@ -1105,6 +1105,18 @@ void SourceBufferStream::Seek(base::TimeDelta timestamp) {
   if (itr == ranges_.end())
     return;
 
+  if (!audio_configs_.empty()) {
+    const auto& config = audio_configs_[(*itr)->GetConfigIdAtTime(seek_dts)];
+    if (config.codec() == kCodecOpus) {
+      DecodeTimestamp preroll_dts = std::max(seek_dts - config.seek_preroll(),
+                                             (*itr)->GetStartTimestamp());
+      if ((*itr)->CanSeekTo(preroll_dts) &&

[chcunningham, 2016/07/22 01:50:28]

FYI: I've taken a conservative approach here. If we cant seek back far enough to
do the proll, or if the configs don't match, we don't preroll (same as today).

[wolenetz, 2016/12/02 00:18:49]

Acknowledged.

+          (*itr)->SameConfigThruRange(preroll_dts, seek_dts)) {
+        seek_dts = preroll_dts;
+      }
+    }
+  }
+
   SeekAndSetSelectedRange(*itr, seek_dts);
   seek_pending_ = false;
 }