[cast] Add optional VideoEncoder method to flush frames.

media/cast/sender/h264_vt_encoder.cc

Index: media/cast/sender/h264_vt_encoder.cc
diff --git a/media/cast/sender/h264_vt_encoder.cc b/media/cast/sender/h264_vt_encoder.cc
index 33923488c51daa96b99847893ea713204588837c..202a8e3d4dea02b946cc516d4b6cc773ca3623f4 100644
--- a/media/cast/sender/h264_vt_encoder.cc
+++ b/media/cast/sender/h264_vt_encoder.cc
@@ -309,6 +309,11 @@ void H264VideoToolboxEncoder::ConfigureSession(
   SetSessionProperty(
       videotoolbox_glue_->kVTCompressionPropertyKey_YCbCrMatrix(),
       kCVImageBufferYCbCrMatrix_ITU_R_709_2);
+  if (video_config.max_number_of_video_buffers_used > 0) {
+    SetSessionProperty(
+        videotoolbox_glue_->kVTCompressionPropertyKey_MaxFrameDelayCount(),
+        video_config.max_number_of_video_buffers_used);
+  }
 }
 
 void H264VideoToolboxEncoder::Teardown() {
@@ -404,6 +409,21 @@ H264VideoToolboxEncoder::CreateVideoFrameFactory() {
       new VideoFrameFactoryCVPixelBufferPoolImpl(pool));
 }
 
+void H264VideoToolboxEncoder::EmitFrames() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  if (!compression_session_) {
+    DLOG(ERROR) << " compression session is null";
+    return;
+  }
+
+  OSStatus status = videotoolbox_glue_->VTCompressionSessionCompleteFrames(
+      compression_session_, CoreMediaGlue::CMTime{0, 0, 0, 0});
+  if (status != noErr) {
+    DLOG(ERROR) << " VTCompressionSessionCompleteFrames failed: " << status;
+  }
+}
+
 bool H264VideoToolboxEncoder::SetSessionProperty(CFStringRef key,
                                                  int32_t value) {
   base::ScopedCFTypeRef<CFNumberRef> cfvalue(