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..7ea0865ffc8504126b2263c7f8d48e51e251eae3 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"));
+  std::string pf1 = metadata->GetPingFreshness("someappid");

[Sorin Jianu, 2016/04/15 20:26:53]

const?

[waffles, 2016/04/15 21:54:20]

Done.

+  EXPECT_FALSE(pf1.empty());
   metadata->SetDateLastRollCall(items, 3386);
   EXPECT_EQ(3386, metadata->GetDateLastRollCall("someappid"));
   EXPECT_EQ(-2, metadata->GetDateLastRollCall("someotherappid"));
+  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) {