[sync] Add backup time in synced device info so that server can flag device

sync/protocol/device_info_specifics.proto

Index: sync/protocol/device_info_specifics.proto
diff --git a/sync/protocol/device_info_specifics.proto b/sync/protocol/device_info_specifics.proto
index 67eb4698a7a970212292b5e8239cdf6d336a4649..474cf4b1c6651230e5be8330e3c55afa058e56a4 100644
--- a/sync/protocol/device_info_specifics.proto
+++ b/sync/protocol/device_info_specifics.proto
@@ -34,4 +34,8 @@ message DeviceInfoSpecifics {
 
   // The Chrome instance's version.  Updated (if necessary) on every startup.
   optional string chrome_version = 5;
+
+  // Last time when pre-sync data on the device was saved. The device can be
+  // restored to state back to this time.

[Nicolas Zea, 2014/06/13 22:36:41]

Mention time format/units

[haitaol1, 2014/06/13 23:28:25]

Done.

+  optional int64 backup_timestamp = 6;
 }