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