[NTP Snippets] Increase default expiry time to 3 days

This matches what our backend (ChromeReader) does.

BUG=643605

Review-Url: https://codereview.chromium.org/2317153003
Cr-Commit-Position: refs/heads/master@{#417256}

components/ntp_snippets/ntp_snippets_service.cc

@@ -68,7 +68,7 @@ const char kFetchingIntervalFallbackParamName[] =
 const int kWifiFetchingHourMin = 6;
 const int kWifiFetchingHourMax = 22;
 
-const int kDefaultExpiryTimeMins = 24 * 60;
+const int kDefaultExpiryTimeMins = 3 * 24 * 60;
 
 base::TimeDelta GetFetchingInterval(const char* switch_name,
                                     const char* param_name,