Cleanup budget_database based on evolving API.

Removed debt, which is no longer needed. Added data structures to replace the
std::pairs. Switched the storage of expiration time from a double to an int.

BUG=617971

Review-Url: https://codereview.chromium.org/2233783002
Cr-Commit-Position: refs/heads/master@{#411100}
4 files changed