[Nearby] Fix integer overflow in expiration scheduler

Using base::Time::Min() for the overdue private certificate expiration
time resulted in integer overflow because base::Time does not use
clamped arithmetic. See crbug/1127026.

Change-Id: I00a898858bf6d2b97aa6a34a37ca2ee90d2c9836
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404363
Commit-Queue: Josh Nohle <nohle@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Auto-Submit: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805949}

chrome/browser/nearby_sharing/scheduling/nearby_share_expiration_scheduler.cc

@@ -4,7 +4,6 @@
 
 #include "chrome/browser/nearby_sharing/scheduling/nearby_share_expiration_scheduler.h"
 
-#include <algorithm>
 #include <utility>
 
 NearbyShareExpirationScheduler::NearbyShareExpirationScheduler(
@@ -32,5 +31,8 @@ NearbyShareExpirationScheduler::TimeUntilRecurringRequest(
   if (!expiration_time)
     return base::nullopt;
 
-  return std::max(base::TimeDelta::FromSeconds(0), *expiration_time - now);
+  if (*expiration_time <= now)
+    return base::TimeDelta::FromSeconds(0);
+
+  return *expiration_time - now;
 }