Add maximum timestamp to the payload.

Added a new field max_timestamp in the protobuf, from now on
update_engine will reject any payload without this field.
If the OS build timestamp is newer than the max_timestamp, the payload
will also be rejected to prevent downgrade.

Bug: 36232423
Test: update_engine_unittests

Merged-In: Ib20f5f35aaf41165013bada02bc8720917358237
Change-Id: Ib20f5f35aaf41165013bada02bc8720917358237
diff --git a/hardware_chromeos.cc b/hardware_chromeos.cc
index 4b0b82f..f0f3ea9 100644
--- a/hardware_chromeos.cc
+++ b/hardware_chromeos.cc
@@ -230,6 +230,11 @@
   return true;
 }
 
+int64_t HardwareChromeOS::GetBuildTimestamp() const {
+  // TODO(senj): implement this in Chrome OS.
+  return 0;
+}
+
 void HardwareChromeOS::LoadConfig(const string& root_prefix, bool normal_mode) {
   brillo::KeyValueStore store;