Add expiration policy, code to check it, and a test.

components/offline_pages/background/offliner_policy.h

Index: components/offline_pages/background/offliner_policy.h
diff --git a/components/offline_pages/background/offliner_policy.h b/components/offline_pages/background/offliner_policy.h
index 5aba61d08c894416f6e395389d63d78221ff273c..169ff411081376e639dcf867e3ac8ca6f9116aad 100644
--- a/components/offline_pages/background/offliner_policy.h
+++ b/components/offline_pages/background/offliner_policy.h
@@ -10,6 +10,7 @@ const int kMaxRetries = 2;
 const int kBackgroundTimeBudgetSeconds = 170;
 const int kSinglePageTimeBudgetSeconds = 120;
 const int kMinimumBatteryPercentageForNonUserRequestOfflining = 50;
+const int kRequestExpirationTimeInSeconds = 60 * 60 * 24 * 7;
 }  // namespace
 
 namespace offline_pages {
@@ -58,7 +59,7 @@ class OfflinerPolicy {
 
   bool UnmeteredNetworkRequiredForNonUserRequestedPage() const { return true; }
 
-  int BatteryPercentageRequiredForUserRequestedPage() const { return 50; }
+  int BatteryPercentageRequiredForUserRequestedPage() const { return 0; }
 
   // This is so low because we require the device to be plugged in and charging.
   // If we decide to allow non-user requested pages when not plugged in, we
@@ -76,6 +77,11 @@ class OfflinerPolicy {
     return kSinglePageTimeBudgetSeconds;
   }
 
+  // How long we allow requests to remain in the system before giving up.
+  int GetRequestExpirationTimeInSeconds() const {
+    return kRequestExpirationTimeInSeconds;
+  }
+
   // How much battery must we have before fetching a page not explicitly
   // requested by the user?
   int GetMinimumBatteryPercentageForNonUserRequestOfflining() const {