[MediaSession] A fix for MediaSessionService routing

Previously, MediaSessionImpl uses the metadata and controls to determine
whether a page uses the MediaSession API. However there comes an issues
that we need to update the routed MediaSession every time when a service
is created/destroyed or its metadata/controls changed, which is a lot of
maintainance work.

This CL simplifies the MediaSessionService routing strategy. It only
checks whether a service is created for a given frame. The rationale is
that a MediaSessionService is created when and only when anything passes
through mojo, which is enough for indicating whether a frame uses
MediaSession API.

BUG=670319

Review-Url: https://codereview.chromium.org/2585243002
Cr-Commit-Position: refs/heads/master@{#439488}

content/browser/media/session/media_session_impl.cc

@@ -575,6 +575,7 @@ bool MediaSessionImpl::AddOneShotPlayer(MediaSessionPlayerObserver* observer,
 
 void MediaSessionImpl::OnServiceCreated(MediaSessionServiceImpl* service) {
   services_[service->GetRenderFrameHost()] = service;
+  UpdateRoutedService();
 }
 
 void MediaSessionImpl::OnServiceDestroyed(MediaSessionServiceImpl* service) {
@@ -617,11 +618,7 @@ void MediaSessionImpl::DidReceiveAction(
 }
 
 bool MediaSessionImpl::IsServiceActiveForRenderFrameHost(RenderFrameHost* rfh) {
-  if (!services_.count(rfh))
-    return false;
-
-  return services_[rfh]->metadata().has_value() ||
-         !services_[rfh]->actions().empty();
+  return services_.find(rfh) != services_.end();
 }
 
 void MediaSessionImpl::UpdateRoutedService() {

content/browser/media/session/media_session_impl_browsertest.cc

@@ -157,7 +157,6 @@ class MediaSessionImplBrowserTest : public content::ContentBrowserTest {
   void EnsureMediaSessionService() {
     mock_media_session_service_.reset(new MockMediaSessionServiceImpl(
         shell()->web_contents()->GetMainFrame()));
-    mock_media_session_service_->SetMetadata(content::MediaMetadata());
   }
 
   void SetPlaybackState(blink::mojom::MediaSessionPlaybackState state) {

content/browser/media/session/media_session_impl_service_routing_unittest.cc

@@ -74,7 +74,10 @@ class MediaSessionImplServiceRoutingTest
  protected:
   void CreateServiceForFrame(TestRenderFrameHost* frame) {
     services_[frame] = base::MakeUnique<MockMediaSessionServiceImpl>(frame);
-    services_[frame]->SetMetadata(MediaMetadata());
+  }
+
+  void DestroyServiceForFrame(TestRenderFrameHost* frame) {
+    services_.erase(frame);
   }
 
   void StartPlayerForFrame(TestRenderFrameHost* frame) {
@@ -133,23 +136,37 @@ TEST_F(MediaSessionImplServiceRoutingTest,
 }
 
 TEST_F(MediaSessionImplServiceRoutingTest,
-       OnlyMainFrameProducesAudioButHasInactiveService) {
-  StartPlayerForFrame(main_frame_);
-
+       OnlyMainFrameProducesAudioButHasDestroyedService) {
   CreateServiceForFrame(main_frame_);
-  services_[main_frame_]->SetMetadata(base::nullopt);
+  StartPlayerForFrame(main_frame_);
+  DestroyServiceForFrame(main_frame_);
 
   ASSERT_EQ(nullptr, ComputeServiceForRouting());
 }
 
 TEST_F(MediaSessionImplServiceRoutingTest,
-       OnlySubFrameProducesAudioButHasInactiveService) {
+       OnlySubFrameProducesAudioButHasDestroyedService) {
+  CreateServiceForFrame(sub_frame_);
   StartPlayerForFrame(sub_frame_);
+  DestroyServiceForFrame(sub_frame_);
+
+  ASSERT_EQ(nullptr, ComputeServiceForRouting());
+}
+
+TEST_F(MediaSessionImplServiceRoutingTest,
+       OnlyMainFrameProducesAudioAndServiceIsCreatedAfterwards) {
+  StartPlayerForFrame(main_frame_);
+  CreateServiceForFrame(main_frame_);
+
+  ASSERT_EQ(services_[main_frame_].get(), ComputeServiceForRouting());
+}
 
+TEST_F(MediaSessionImplServiceRoutingTest,
+       OnlySubFrameProducesAudioAndServiceIsCreatedAfterwards) {
+  StartPlayerForFrame(sub_frame_);
   CreateServiceForFrame(sub_frame_);
-  services_[sub_frame_]->SetMetadata(base::nullopt);
 
-  ASSERT_EQ(nullptr, ComputeServiceForRouting());
+  ASSERT_EQ(services_[sub_frame_].get(), ComputeServiceForRouting());
 }
 
 TEST_F(MediaSessionImplServiceRoutingTest,