Added browser tests to test the BudgetAPI. Also added BudgetAPI to ExperimentalWebPlatformFeatures.

Added browser tests to test the BudgetAPI. These test the getBudget()
and reserve() methods from Blink into the chrome layer. The correct
functioning is then tested by a direct call to BudgetManager::Consume.

Also added BudgetAPI to ExperimentalWebPlatformFeatures.

BUG=617971

Review-Url: https://codereview.chromium.org/2370103003
Cr-Commit-Position: refs/heads/master@{#422431}
12 files changed