Implement ping_freshness for update_client.

components/update_client/persisted_data_unittest.cc

Index: components/update_client/persisted_data_unittest.cc
diff --git a/components/update_client/persisted_data_unittest.cc b/components/update_client/persisted_data_unittest.cc
index d89060f2cda652e50c702d47f83cce97acca2da4..dfe242c2a61f37e466e4341d26cc54fa186d6e49 100644
--- a/components/update_client/persisted_data_unittest.cc
+++ b/components/update_client/persisted_data_unittest.cc
@@ -22,9 +22,15 @@ TEST(PersistedDataTest, Simple) {
   metadata->SetDateLastRollCall(items, 3383);
   EXPECT_EQ(3383, metadata->GetDateLastRollCall("someappid"));
   EXPECT_EQ(-2, metadata->GetDateLastRollCall("someotherappid"));
+  const std::string pf1 = metadata->GetPingFreshness("someappid");
+  EXPECT_FALSE(pf1.empty());
   metadata->SetDateLastRollCall(items, 3386);
   EXPECT_EQ(3386, metadata->GetDateLastRollCall("someappid"));
   EXPECT_EQ(-2, metadata->GetDateLastRollCall("someotherappid"));
+  const std::string pf2 = metadata->GetPingFreshness("someappid");
+  EXPECT_FALSE(pf2.empty());
+  // The following has a 1 / 2^128 chance of being flaky.
+  EXPECT_NE(pf1, pf2);
 }
 
 TEST(PersistedDataTest, SharedPref) {