Compression data such as the aggregate number of bytes downloaded and the aggregate number of bytes that would have been downloaded if not for the compression proxy are stored in prefs. Separately, the data reduction proxy code provides a leveldb implementation (for the desktop extension, primarily) to store data use and savings broken down in 15 minute increments. The data reduction proxy should deprecate its use of prefs for storing such data in favor of the database.

In addition, to work within typical Android device storage constraints, we may want to use larger intervals for the data use breakdown.

Also, the current implementation of DRPCompressionStats is very confusing and should be simplified. I would expect a public API like this:

class DataUseStore {
 public:
  // Descriptor includes things like request type, data use group, 
  // mime type, and host, or could be broken out.
  void UpdateDataUse(int64_t transferred_size,
                     int64_t original_size,
                     const DataUseDescriptor& description);

  AggregateDataUse GetAggregateDataUse();

  // Query could include host or interval, or could be broken out.
  DataUseList GetDataUse(const DataUseQuery& query);

  void ClearDataUse();
};