Make MediaSession a runtime-enabled feature on Desktop.

content/browser/web_contents/web_contents_impl.h

Index: content/browser/web_contents/web_contents_impl.h
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index a5f632ec0c7a29c5ab55c488c22390854f9251c9..ea96c822207cb1a7c7f5be2be132f7eb4b7285de 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -382,12 +382,12 @@ class CONTENT_EXPORT WebContentsImpl
   void HasManifest(const HasManifestCallback& callback) override;
   void ExitFullscreen(bool will_cause_resize) override;
   void ResumeLoadingCreatedWebContents() override;
-#if defined(OS_ANDROID)
   void OnMediaSessionStateChanged();
   void ResumeMediaSession() override;
   void SuspendMediaSession() override;
   void StopMediaSession() override;
 
+#if defined(OS_ANDROID)
   base::android::ScopedJavaLocalRef<jobject> GetJavaWebContents() override;
   virtual WebContentsAndroid* GetWebContentsAndroid();
 #elif defined(OS_MACOSX)