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

chrome/browser/push_messaging/background_budget_service.cc

Index: chrome/browser/push_messaging/background_budget_service.cc
diff --git a/chrome/browser/push_messaging/background_budget_service.cc b/chrome/browser/push_messaging/background_budget_service.cc
index cbcb169f211be16066eabddf007ed853b0cfcae3..8589cd510c1f9a9bd0ff4be1689425c23922c1c1 100644
--- a/chrome/browser/push_messaging/background_budget_service.cc
+++ b/chrome/browser/push_messaging/background_budget_service.cc
@@ -129,6 +129,12 @@ double BackgroundBudgetService::GetBudget(const GURL& origin) {
   base::Time now = clock_->Now();
   base::TimeDelta elapsed = now - base::Time::FromDoubleT(last_updated_msec);
 
+  // The user can set their clock backwards, so if the last updated time is in
+  // the future, don't update the budget based on elapsed time. Eventually the
+  // clock will reach the future, and the budget calculations will catch up.
+  if (elapsed.InMicroseconds() < 0)

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

Conversely, the clock may jump forwards a lot, e.g. if the clock periodically
resets to 1970 then is fixed by ntp, or if the user jumps back and forwards
excessively to cheat at a game.

It would be unfortunate to remove all budget whenever that happens (which may be
daily for some users). If time jumps forward more than ~1 week (1 month?)
without Chrome getting run at all, it's likely that time didn't really advance
that much. Ideally Chrome would centrally detect such jumps in time, though I
haven't yet encountered code for that.

Add a TODO for this?

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

Done.

+    return old_budget;
+
   // For each time period that elapses, calculate the carryover ratio as the
   // ratio of time remaining in our max period to the total period.
   // The carryover component is then the old budget multiplied by the ratio.