Add a mechanism for collecting Rappor samples on a daily interval.

components/rappor/rappor_pref_names.cc

Index: components/rappor/rappor_pref_names.cc
diff --git a/components/rappor/rappor_pref_names.cc b/components/rappor/rappor_pref_names.cc
index ff444efd2ba19323ff04e911ae834a5c89a3ece9..b879c4bd2b3ad13c33faee5e86914e0204bf35f0 100644
--- a/components/rappor/rappor_pref_names.cc
+++ b/components/rappor/rappor_pref_names.cc
@@ -13,6 +13,9 @@ const char kRapporCohortDeprecated[] = "rappor.cohort";
 // A randomly generated number, which determines cohorts data is reported for.
 const char kRapporCohortSeed[] = "rappor.cohort_seed";
 
+// Timestamp of the last time we sampled daily metrics.
+const char kRapporLastDailySample[] = "rappor.last_daily_sample";
+
 // A base-64 encoded, randomly generated byte string, which is used as a seed
 // for redacting collected data.
 // Important: This value should remain secret at the client, and never be