Rename BackgroundBudgetService to BudgetManager

It's clear that there's only going to be a single budget service, so "Background" didn't
make sense anymore. Also, with an interface named BudgetService and a Mojo service named
BudgetService, we needed something other than another BudgetService. The component does
manage calls from the Blink and browser layers into the guts of the BudgetDatabase, so
BudgetManager makes sense.

BUG=617971

Review-Url: https://codereview.chromium.org/2243813002
Cr-Commit-Position: refs/heads/master@{#412186}
14 files changed