[Video] Pause videos with no audio when in the background.

Pause video-only players when they go to background and continue playback when they go into foreground and hasn't been paused explicitly in the mean time.
Behind the experiment for optimizing background video playback.

BUG=671381
TEST=Manual

Review-Url: https://codereview.chromium.org/2567833002
Cr-Commit-Position: refs/heads/master@{#442110}

media/blink/webmediaplayer_impl.cc

@@ -205,6 +205,7 @@ WebMediaPlayerImpl::WebMediaPlayerImpl(
       opaque_(false),
       playback_rate_(0.0),
       paused_(true),
+      paused_when_hidden_(false),
       seeking_(false),
       pending_suspend_resume_cycle_(false),
       ended_(false),
@@ -448,6 +449,9 @@ void WebMediaPlayerImpl::pause() {
   // paused state.
   paused_ = true;
 
+  // No longer paused because it was hidden.
+  paused_when_hidden_ = false;
+
 #if defined(OS_ANDROID)  // WMPI_CAST
   if (isRemote()) {
     cast_impl_.pause();
@@ -1342,12 +1346,21 @@ void WebMediaPlayerImpl::OnVideoOpacityChange(bool opaque) {
 void WebMediaPlayerImpl::OnHidden() {
   DCHECK(main_task_runner_->BelongsToCurrentThread());
 
-  if (IsBackgroundVideoTrackOptimizationEnabled())
-    selectedVideoTrackChanged(nullptr);
-
   if (watch_time_reporter_)
     watch_time_reporter_->OnHidden();
 
+  if (IsBackgroundVideoTrackOptimizationEnabled()) {
+    if (ShouldPauseWhenHidden()) {
+      // OnPause() will set |paused_when_hidden_| to false and call
+      // UpdatePlayState(), so set the flag to true after and then return.
+      OnPause();
+      paused_when_hidden_ = true;
+      return;
+    }
+
+    selectedVideoTrackChanged(nullptr);
+  }
+
   UpdatePlayState();
 
   // Schedule suspended playing media to be paused if the user doesn't come back
@@ -1365,10 +1378,17 @@ void WebMediaPlayerImpl::OnShown() {
       base::Bind(&VideoFrameCompositor::SetForegroundTime,
                  base::Unretained(compositor_), base::TimeTicks::Now()));
 
-  if (IsBackgroundVideoTrackOptimizationEnabled() &&
-      client_->hasSelectedVideoTrack()) {
-    WebMediaPlayer::TrackId trackId = client_->getSelectedVideoTrackId();
-    selectedVideoTrackChanged(&trackId);
+  if (IsBackgroundVideoTrackOptimizationEnabled()) {
+    if (paused_when_hidden_) {
+      paused_when_hidden_ = false;
+      OnPlay();  // Calls UpdatePlayState() so return afterwards.
+      return;
+    }
+
+    if (client_->hasSelectedVideoTrack()) {
+      WebMediaPlayer::TrackId trackId = client_->getSelectedVideoTrackId();
+      selectedVideoTrackChanged(&trackId);
+    }
   }
 
   must_suspend_ = false;
@@ -2051,4 +2071,13 @@ void WebMediaPlayerImpl::ActivateViewportIntersectionMonitoring(bool activate) {
   client_->activateViewportIntersectionMonitoring(activate);
 }
 
+bool WebMediaPlayerImpl::ShouldPauseWhenHidden() const {
+#if defined(OS_ANDROID)  // WMPI_CAST
+  if (isRemote())
+    return false;
+#endif  // defined(OS_ANDROID)  // WMPI_CAST
+
+  return hasVideo() && !hasAudio();
+}
+
 }  // namespace media

media/blink/webmediaplayer_impl.h

@@ -368,6 +368,9 @@ class MEDIA_BLINK_EXPORT WebMediaPlayerImpl
   // is intended for android.
   bool DoesOverlaySupportMetadata() const;
 
+  // Whether the media should be paused when hidden.
+  bool ShouldPauseWhenHidden() const;
+
   blink::WebLocalFrame* frame_;
 
   // The playback state last reported to |delegate_|, to avoid setting duplicate
@@ -431,6 +434,10 @@ class MEDIA_BLINK_EXPORT WebMediaPlayerImpl
   bool paused_;
   base::TimeDelta paused_time_;
 
+  // Set if paused automatically when hidden and need to resume when visible.
+  // Reset if paused for any other reason.
+  bool paused_when_hidden_;
+
   // Set when starting, seeking, and resuming (all of which require a Pipeline
   // seek). |seek_time_| is only valid when |seeking_| is true.
   bool seeking_;