Framework and tests for WorkerNavigatorBudget

This CL provides a Worker entrypoint for the BudgetAPI. The WorkerNavigator
partial interface provides a BudgetService object, which is the same for
both Navigator and WorkerNavigator.

BUG=617971

Review-Url: https://codereview.chromium.org/2273743002
Cr-Commit-Position: refs/heads/master@{#414579}
11 files changed