Updated the return of GetBudget to a sequence

The proposed spec for BudgetAPI defines the return value for getBudget
to be a Promise that resolves with a sequence<BudgetState> while the
current interface used a FrozenArray.

This patch updates the code to match the proposed spec. The implementation
works with either version, and this is covered by existing LayoutTests.

BUG=672112

Review-Url: https://codereview.chromium.org/2571573002
Cr-Commit-Position: refs/heads/master@{#438127}

third_party/WebKit/Source/modules/budget/BudgetService.idl

@@ -12,6 +12,6 @@ enum OperationType {
     RuntimeEnabled=Budget
 ] interface BudgetService {
     [CallWith=ScriptState] Promise<double> getCost(OperationType operation);
-    [CallWith=ScriptState] Promise<FrozenArray<BudgetState>> getBudget();
+    [CallWith=ScriptState] Promise<sequence<BudgetState>> getBudget();
     [CallWith=ScriptState] Promise<boolean> reserve(OperationType operation);
 };