Add utility method to read the client id from a writeback file.
Add utility method to read the client id from a writeback file.

dunkhong/grr,dunkhong/grr,google/grr,google/grr,google/grr,dunkhong/grr,google/grr,dunkhong/grr,google/grr,google/grr,google/grr,dunkhong/grr,dunkhong/grr,dunkhong/grr