Send empty video frames when screen is not changing.

Before r272790 the host was sending empty packets 30 times a second
when nothing is changing on the screen. After that change and r281677
the frequency was decreased to 5 frame per second. It looks like those
changes degraded performance for some users. This change partially
restores previous behavior. Empty packets are send for each captured
frame even when nothing is changing on the screen. Keep-alive packets
are sent every 200ms when there are no other video packets, e.g. when
the video stream is paused.

Review URL: https://codereview.chromium.org/477103004

Cr-Commit-Position: refs/heads/master@{#290028}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290028 0039d316-1c4b-4281-b951-d872f2087c98

remoting/host/video_scheduler.cc

@@ -31,10 +31,10 @@ namespace remoting {
 // TODO(hclam): Move this value to CaptureScheduler.
 static const int kMaxPendingFrames = 2;
 
-// Interval between empty keep-alive frames. These frames are sent only
-// when there are no real video frames being sent. To prevent PseudoTCP from
-// resetting congestion window this value must be smaller than the minimum
-// RTO used in PseudoTCP, which is 250ms.
+// Interval between empty keep-alive frames. These frames are sent only when the
+// stream is paused or inactive for some other reason (e.g. when blocked on
+// capturer). To prevent PseudoTCP from resetting congestion window this value
+// must be smaller than the minimum RTO used in PseudoTCP, which is 250ms.
 static const int kKeepAlivePacketIntervalMs = 200;
 
 static bool g_enable_timestamps = false;
@@ -374,11 +374,15 @@ void VideoScheduler::EncodeFrame(
     base::TimeTicks timestamp) {
   DCHECK(encode_task_runner_->BelongsToCurrentThread());
 
-  // Drop the frame if there were no changes.
+  // If there is nothing to encode then send an empty packet.
   if (!frame || frame->updated_region().is_empty()) {
     capture_task_runner_->DeleteSoon(FROM_HERE, frame.release());
-    capture_task_runner_->PostTask(
-        FROM_HERE, base::Bind(&VideoScheduler::FrameCaptureCompleted, this));
+    scoped_ptr<VideoPacket> packet(new VideoPacket());
+    packet->set_client_sequence_number(sequence_number);
+    network_task_runner_->PostTask(
+        FROM_HERE,
+        base::Bind(
+            &VideoScheduler::SendVideoPacket, this, base::Passed(&packet)));
     return;
   }