Update //third_party/protobuf to version 3.

third_party/protobuf/examples/README.txt

+This directory contains example code that uses Protocol Buffers to manage an
+address book.  Two programs are provided, each with three different
+implementations, one written in each of C++, Java, and Python.  The add_person
+example adds a new person to an address book, prompting the user to input
+the person's information.  The list_people example lists people already in the
+address book.  The examples use the exact same format in all three languages,
+so you can, for example, use add_person_java to create an address book and then
+use list_people_python to read it.
+
+You must install the protobuf package before you can build these.
+
+To build all the examples (on a unix-like system), simply run "make".  This
+creates the following executable files in the current directory:
+  add_person_cpp     list_people_cpp
+  add_person_java    list_people_java
+  add_person_python  list_people_python
+
+If you only want to compile examples in one language, use "make cpp"*,
+"make java", or "make python".
+
+All of these programs simply take an address book file as their parameter.
+The add_person programs will create the file if it doesn't already exist.
+
+These examples are part of the Protocol Buffers tutorial, located at:
+  https://developers.google.com/protocol-buffers/docs/tutorials
+
+* Note that on some platforms you may have to edit the Makefile and remove
+"-lpthread" from the linker commands (perhaps replacing it with something else).
+We didn't do this automatically because we wanted to keep the example simple.
+
+## Go ##
+
+The Go example requires a plugin to the protocol buffer compiler, so it is not
+build with all the other examples.  See:
+  https://github.com/golang/protobuf
+for more information about Go protocol buffer support.
+
+First, install the the Protocol Buffers compiler (protoc).
+Then, install the Go Protocol Buffers plugin
+($GOPATH/bin must be in your $PATH for protoc to find it):
+  go get github.com/golang/protobuf/protoc-gen-go
+
+Build the Go samples in this directory with "make go".  This creates the
+following executable files in the current directory:
+  add_person_go      list_people_go
+To run the example:
+  ./add_person_go addressbook.data
+to add a person to the protocol buffer encoded file addressbook.data.  The file
+is created if it does not exist.  To view the data, run:
+  ./list_people_go addressbook.data
+
+Observe that the C++, Python, and Java examples in this directory run in a
+similar way and can view/modify files created by the Go example and vice
+versa.