Bug 1580623 - P2: Fire seek{for,back}ward action handler if it's set r=alwu

Depends on D45459

Differential Revision: https://phabricator.services.mozilla.com/D66619

--HG--
extra : moz-landing-system : lando

dom/media/mediacontrol/PlaybackController.cpp

@@ -66,15 +66,19 @@ void PlaybackController::Pause() {
 }
 
 void PlaybackController::SeekBackward() {
-  // TODO : use media session's action handler if it exists. MediaSessionAction
-  // doesn't support `seekbackward` yet.
-  return;
+  const MediaSessionAction action = MediaSessionAction::Seekbackward;
+  if (RefPtr<MediaSession> session = GetMediaSession();
+      session && session->IsSupportedAction(action)) {
+    session->NotifyHandler(action);
+  }
 }
 
 void PlaybackController::SeekForward() {
-  // TODO : use media session's action handler if it exists. MediaSessionAction
-  // doesn't support `seekforward` yet.
-  return;
+  const MediaSessionAction action = MediaSessionAction::Seekforward;
+  if (RefPtr<MediaSession> session = GetMediaSession();
+      session && session->IsSupportedAction(action)) {
+    session->NotifyHandler(action);
+  }
 }
 
 void PlaybackController::PreviousTrack() {

dom/media/mediasession/test/MediaSessionTestUtils.js

@@ -1,6 +1,8 @@
 const gMediaSessionActions = [
   "play",
   "pause",
+  "seekbackward",
+  "seekforward",
   "previoustrack",
   "nexttrack",
   "stop",

dom/media/mediasession/test/file_media_session.html

@@ -11,6 +11,8 @@
 const MediaSessionActions = [
   "play",
   "pause",
+  "seekbackward",
+  "seekforward",
   "previoustrack",
   "nexttrack",
   "stop",