This CL converts the GetBudget and StoreBudget calls on the
BackgroundBudgetService to be asynchonous. This is a step on the path to
converting BackgroundBudgetService budget storage to use a LevelDB storage
instead of prefs storage, which will require some of the budget service's
methods to run on a thread other than the UI thread.

BUG=617971

Review-Url: https://codereview.chromium.org/2058523003
Cr-Commit-Position: refs/heads/master@{#400707}
5 files changed