media: Implement MediaDrmStorageImpl with tests

components/cdm/browser/media_drm_storage_impl.h

Index: components/cdm/browser/media_drm_storage_impl.h
diff --git a/components/cdm/browser/media_drm_storage_impl.h b/components/cdm/browser/media_drm_storage_impl.h
index c71a90d1cf7a15b3ed39aca23279b231a5bd9a0c..a581cb6f04d69a532b4e167c19b12eaa0be2e91a 100644
--- a/components/cdm/browser/media_drm_storage_impl.h
+++ b/components/cdm/browser/media_drm_storage_impl.h
@@ -5,6 +5,7 @@
 #ifndef COMPONENTS_CDM_BROWSER_MEDIA_DRM_STORAGE_IMPL_H_
 #define COMPONENTS_CDM_BROWSER_MEDIA_DRM_STORAGE_IMPL_H_
 
+#include "base/threading/thread_checker.h"
 #include "content/public/browser/render_frame_host.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "media/mojo/interfaces/media_drm_storage.mojom.h"
@@ -53,12 +54,15 @@ class MediaDrmStorageImpl final : public media::mojom::MediaDrmStorage,
   void DidFinishNavigation(content::NavigationHandle* navigation_handle) final;
 
  private:
+  base::ThreadChecker thread_checker_;
+
   // Stops observing WebContents and delete |this|.
   void Close();
 
   content::RenderFrameHost* const render_frame_host_ = nullptr;
   PrefService* const pref_service_ = nullptr;
   const url::Origin origin_;
+  const std::string origin_string_;
   bool initialized_ = false;
 
   mojo::Binding<media::mojom::MediaDrmStorage> binding_;