Follow this design doc for creating leveldb scopes, and put that implementation in content/browser/indexed_db/leveldb/

https://docs.google.com/document/u/1/d/16_igCI15Gfzb6UYqeuJTmJPrzEtawz6Y1tVOKNtYgiU/edit#


Known unknowns
* Which style of undo log will work
* If undo log is saved in leveldb, if there is a worst-case for adding/deleting where leveldb compacts slower than we save data.
* How much the performance will suffer.
* How much memory will be saved.

Depending on the choice of undo log, deferred deletion could be a part of this feature or done separately