Add UMA metrics for detecting and reporting update certificate changes.

LibcurlHttpFetcher checks if the update server certificate has changed
since last update, and stores an eventual report in prefs. UpdateCheckScheduler
submits to UMA reports from the previous update.

BUG=chromium-os:19842
TEST=Included unittest for the new class and tested locally on an Alex device,
since we need to verify against the actual server certificates.

Change-Id: I5bee5d648982cd7618db09b67d5bff377eaa1fc1
Reviewed-on: http://gerrit.chromium.org/gerrit/7565
Reviewed-by: Bruno Pontes Soares Rocha <bpontes@chromium.org>
Tested-by: Bruno Pontes Soares Rocha <bpontes@chromium.org>
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
diff --git a/prefs_interface.h b/prefs_interface.h
index ddebb3b..624005e 100644
--- a/prefs_interface.h
+++ b/prefs_interface.h
@@ -9,6 +9,8 @@
 
 namespace chromeos_update_engine {
 
+extern const char kPrefsCertificateReportToSendDownload[];
+extern const char kPrefsCertificateReportToSendUpdate[];
 extern const char kPrefsDeltaUpdateFailures[];
 extern const char kPrefsLastActivePingDay[];
 extern const char kPrefsLastRollCallPingDay[];
@@ -16,6 +18,7 @@
 extern const char kPrefsPreviousVersion[];
 extern const char kPrefsResumedUpdateFailures[];
 extern const char kPrefsUpdateCheckResponseHash[];
+extern const char kPrefsUpdateServerCertificate[];
 extern const char kPrefsUpdateStateNextDataOffset[];
 extern const char kPrefsUpdateStateNextOperation[];
 extern const char kPrefsUpdateStateSHA256Context[];