Convert autofill messages to use the new IPC macros (again).

Make FormField be a struct rather than a class per c++ standards.
This makes automatic generation of IPC messages much easier.
Convert autofill messsage to automatic generation in the process.
Attempt #2: fix build on chromeos, see http://codereview.chromium.org/6633001
for original CL.


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=77307