DLP: Send black frame when screen capture is paused.

When confidential content appears in the capture area and
screen capture is being paused, a black frame should be sent
to override the last sent capture frame.

Bug: 1134566
Change-Id: I20c2859cefcc0166ae86b08cde02e5b1b8835dbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521737Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825296}

third_party/blink/public/web/modules/mediastream/media_stream_video_source.h

@@ -102,7 +102,8 @@ class BLINK_MODULES_EXPORT MediaStreamVideoSource
   // verified by checking that the IsRunning() method returns true.
   // Any attempt to invoke StopForRestart() before the source has started
   // results in no action and |callback| invoked with INVALID_STATE.
-  void StopForRestart(RestartCallback callback);
+  // If |send_black_frame| is set, an additional black frame will be sent.
+  void StopForRestart(RestartCallback callback, bool send_black_frame = false);
 
   // Tries to restart a source that was previously temporarily stopped using the
   // supplied |new_format|. This method can be invoked only after a successful

third_party/blink/renderer/modules/mediastream/media_stream_video_source.cc

@@ -198,7 +198,8 @@ void MediaStreamVideoSource::ReconfigureTrack(
   UpdateTrackSettings(track, adapter_settings);
 }
 
-void MediaStreamVideoSource::StopForRestart(RestartCallback callback) {
+void MediaStreamVideoSource::StopForRestart(RestartCallback callback,
+                                            bool send_black_frame) {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
   if (state_ != STARTED) {
     Thread::Current()->GetTaskRunner()->PostTask(
@@ -211,6 +212,21 @@ void MediaStreamVideoSource::StopForRestart(RestartCallback callback) {
   track_adapter_->StopFrameMonitoring();
   state_ = STOPPING_FOR_RESTART;
   restart_callback_ = std::move(callback);
+
+  if (send_black_frame) {
+    const base::Optional<gfx::Size> source_size =
+        track_adapter_->source_frame_size();
+    scoped_refptr<media::VideoFrame> black_frame =
+        media::VideoFrame::CreateBlackFrame(
+            source_size.has_value() ? *source_size
+                                    : gfx::Size(kDefaultWidth, kDefaultHeight));
+    PostCrossThreadTask(
+        *io_task_runner(), FROM_HERE,
+        CrossThreadBindOnce(&VideoTrackAdapter::DeliverFrameOnIO,
+                            track_adapter_, black_frame,
+                            base::TimeTicks::Now()));
+  }
+
   StopSourceForRestartImpl();
 }
 

third_party/blink/renderer/modules/mediastream/user_media_processor.cc

@@ -1228,7 +1228,8 @@ void UserMediaProcessor::OnDeviceRequestStateChange(
   }
   if (new_state == mojom::blink::MediaStreamStateChange::PAUSE) {
     if (video_source->IsRunning()) {
-      video_source->StopForRestart(base::DoNothing());
+      video_source->StopForRestart(base::DoNothing(),
+                                   /*send_black_frame=*/true);
     }
   } else if (new_state == mojom::blink::MediaStreamStateChange::PLAY) {
     if (video_source->IsStoppedForRestart()) {

third_party/blink/renderer/modules/mediastream/video_track_adapter.cc

@@ -413,6 +413,7 @@ bool VideoTrackAdapter::VideoFrameResolutionAdapter::MaybeDropFrame(
   if (settings_.max_frame_rate() == 0.0f ||
       (source_frame_rate > 0 &&
        source_frame_rate <= settings_.max_frame_rate())) {
+    last_time_stamp_ = frame.timestamp();
     return false;
   }
 

third_party/blink/renderer/modules/mediastream/video_track_adapter.h

@@ -95,6 +95,10 @@ class MODULES_EXPORT VideoTrackAdapter
                                    const VideoTrackAdapterSettings& settings,
                                    gfx::Size* desired_size);
 
+  base::Optional<gfx::Size> source_frame_size() const {
+    return source_frame_size_;
+  }
+
  private:
   virtual ~VideoTrackAdapter();
   friend class WTF::ThreadSafeRefCounted<VideoTrackAdapter>;