Add the future storage framework for the BudgetDatabase.

chrome/browser/budget_service/budget.proto

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+syntax = "proto2";
+
+package budget_service;
+
+// Chrome requires this.
+option optimize_for = LITE_RUNTIME;
+
+// Next available id: 3
+message Budget {
+  // How much budget the origin needs to accumulate before it will be allowed to
+  // perform another budget action.
+  optional double debt = 1;
+
+  // The sequence of budget chunks and their expiration times.
+  repeated BudgetChunk budget = 2;
+}
+
+// Next available id: 3
+message BudgetChunk {
+  // The amount of budget remaining in this chunk.
+  optional double budget_amount = 1;
+
+  // The timestamp when the budget expires. Format is double to integrate with

[Peter Beverloo, 2016/06/30 13:12:46]

Yes, but what does the `double` contain? Milliseconds since the UNIX epoch?
Something else?

[harkness, 2016/07/01 12:29:54]

Done.

+  // Site Engagement.
+  optional double expiration_timestamp = 2;
+}