Summary:
When db_env_ != backup_env_, InsertPathnameToSizeBytes() would
use the wrong Env during backup creation. This happened because this function
used backup_env_ instead of db_env_ to get WAL/data file sizes.
This diff adds an argument to InsertPathnameToSizeBytes() indicating which Env
to use.
Test Plan: ran @anirbanb's BackupTestTool
Reviewers: sdong
Reviewed By: sdong
Subscribers: andrewkr, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D57159