media: Implement MediaDrmStorageImpl with tests

media/base/android/media_drm_storage.h

Index: media/base/android/media_drm_storage.h
diff --git a/media/base/android/media_drm_storage.h b/media/base/android/media_drm_storage.h
index 96bc69e7a0b292cec424ff02ae7a43206e9c1d06..c21a41865687ddc29f6b24ba966adbd707c4fa28 100644
--- a/media/base/android/media_drm_storage.h
+++ b/media/base/android/media_drm_storage.h
@@ -68,6 +68,10 @@ class MEDIA_EXPORT MediaDrmStorage {
       LoadPersistentSessionCB load_persistent_session_cb) = 0;
 
   // Removes the persistent session info for |session_id| from the storage.
+  // If the session for |session_id| exists in the storage, it is removed.
+  // Otherwise, this call is a no-op. In both cases, the result will be true.
+  // The result will be false on other unexpected errors, e.g. connection error
+  // to the storage backend.
   virtual void RemovePersistentSession(const std::string& session_id,
                                        ResultCB result_cb) = 0;