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..64bc05f2c9b0d9b6f1f33ca864232bc6b3a07c02 100644
--- a/media/base/android/media_drm_storage.h
+++ b/media/base/android/media_drm_storage.h
@@ -68,6 +68,8 @@ 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 and
+  // the result will be true. Otherwise, the result will be false.

[yucliu1, 2017/04/04 06:50:00]

If session_id doesn't exist, this should be true, because the result is the same
as 'removed'

[xhwang, 2017/04/06 22:00:51]

Done. No matter what the result is, it seems there's nothing the caller can do.

   virtual void RemovePersistentSession(const std::string& session_id,
                                        ResultCB result_cb) = 0;