Added code to make sure that the budget service does sane things if the clock rolls backwards.

chrome/browser/push_messaging/background_budget_service_unittest.cc

Index: chrome/browser/push_messaging/background_budget_service_unittest.cc
diff --git a/chrome/browser/push_messaging/background_budget_service_unittest.cc b/chrome/browser/push_messaging/background_budget_service_unittest.cc
index 75c34893bf5056227f33e8e679a9d1f329267d4e..187a80a0234d6b07a05362782151640c2237902f 100644
--- a/chrome/browser/push_messaging/background_budget_service_unittest.cc
+++ b/chrome/browser/push_messaging/background_budget_service_unittest.cc
@@ -214,3 +214,28 @@ TEST_F(BackgroundBudgetServiceTest, GetBudgetInvalidBudget) {
 
   EXPECT_DOUBLE_EQ(budget, kTestSES);
 }
+
+TEST_F(BackgroundBudgetServiceTest, GetBudgetNegativeTime) {
+  // Manually construct a BackgroundBudgetService with a clock that the test
+  // can control so that we can fast forward in time.
+  BackgroundBudgetService* service = GetService();
+  base::SimpleTestClock* clock = SetClockForTesting();
+  base::Time starting_time = clock->Now();
+
+  // Set initial SES and budget values.
+  const GURL origin(kTestOrigin);
+  SetSiteEngagementScore(origin, kTestSES);
+  service->StoreBudget(origin, kTestBudget);
+  double budget = 0.0;

[johnme, 2016/06/01 18:24:38]

Nit: declare this variable as `double budget = service->GetBudget(origin);`
below, rather than giving it a default value that can never be used.

[harkness, 2016/06/02 10:06:34]

Done.

+
+  // Move time forward an hour and get the budget.
+  clock->SetNow(starting_time + base::TimeDelta::FromHours(1));
+  budget = service->GetBudget(origin);

[johnme, 2016/06/01 18:24:38]

Nit: add an assert that this budget is different from kTestBudget (since the
comment below says we want to test it matches the budget of the most foward
time)

[harkness, 2016/06/02 10:06:34]

Done.

+  service->StoreBudget(origin, budget);
+
+  // Now move time backwards a day and make sure that the current
+  // budget matches the budget of the most foward time.
+  clock->SetNow(starting_time - base::TimeDelta::FromDays(1));
+  double back_budget = service->GetBudget(origin);
+  EXPECT_NEAR(budget, back_budget, 0.01);
+}