Update the protocol buffer compiler plugin to use Int64 from the fixnum package

The 64-bit integers in protocol buffer messages now use the Int64
object from the fixnum package for representing 64-bit integers
instead of an 8-byte binary buffer.

When generating JSON the 64-bit integers in the 53-bit range are
written as integers. Larger integeres are still written as strings.

R=sigmund@google.com
BUG=

Review URL: https://chromiumcodereview.appspot.com//25708003