[cast] Add optional VideoEncoder method to flush frames.

media/cast/sender/video_sender.cc

 // Copyright 2014 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 "media/cast/sender/video_sender.h"
 
 #include <algorithm>
 #include <cstring>
 
 #include "base/bind.h"
@@ skipping 168 matching lines @@
 
   if (ShouldDropNextFrame(duration_added_by_next_frame)) {
     base::TimeDelta new_target_delay = std::min(
         current_round_trip_time_ * kRoundTripsNeeded +
         base::TimeDelta::FromMilliseconds(kConstantTimeMs),
         max_playout_delay_);
     if (new_target_delay > target_playout_delay_) {
       VLOG(1) << "New target delay: " << new_target_delay.InMilliseconds();
       playout_delay_change_cb_.Run(new_target_delay);
     }
+
+    // Some encoder implementations have a frame window for analysis. Since we
+    // are dropping this frame, unless we instruct the encoder to flush all the
+    // frames that have been enqueued for encoding, frames_in_encoder_ and
+    // last_enqueued_frame_reference_time_ will never be updated and we will
+    // drop every subsequent frame for the rest of the session.
+    video_encoder_->EmitFrames();

[miu, 2015/01/21 23:24:27]

Some questions:

Will the video encoder emit frames if EmitFrames() is never called?  

Will it do so in a timely manner so as to not cause ShouldDropNextFrame() to
trigger because of a "hold-up" within the encoder?

Is this feature of the toolbox encoder meant for real-time use?  Or, is it only
meant for the batch/file streaming use cases?

Are there any other toolbox config options to distinguish that the encoder is
being used for the real-time use case?

+
     return;
   }
 
   uint32 bitrate = congestion_control_->GetBitrate(
         reference_time + target_playout_delay_, target_playout_delay_);
   if (bitrate != last_bitrate_) {
     video_encoder_->SetBitRate(bitrate);
     last_bitrate_ = bitrate;
   }
 
@@ skipping 52 matching lines @@
   DCHECK_GE(frames_in_encoder_, 0);
 
   duration_in_encoder_ =
       last_enqueued_frame_reference_time_ - encoded_frame->reference_time;
 
   SendEncodedFrame(encoder_bitrate, encoded_frame.Pass());
 }
 
 }  // namespace cast
 }  // namespace media