Update //third_party/protobuf to version 3.

third_party/protobuf/java/README.md

+Protocol Buffers - Google's data interchange format
+===================================================
+
+[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf)
+
+Copyright 2008 Google Inc.
+
+This directory contains the Java Protocol Buffers runtime library.
+
+Installation - With Maven
+=========================
+
+The Protocol Buffers build is managed using Maven.  If you would
+rather build without Maven, see below.
+
+1) Install Apache Maven if you don't have it:
+
+     http://maven.apache.org/
+
+2) Build the C++ code, or obtain a binary distribution of protoc.  If
+   you install a binary distribution, make sure that it is the same
+   version as this package.  If in doubt, run:
+
+     $ protoc --version
+
+   You will need to place the protoc executable in ../src.  (If you
+   built it yourself, it should already be there.)
+
+3) Run the tests:
+
+     $ mvn test
+
+   If some tests fail, this library may not work correctly on your
+   system.  Continue at your own risk.
+
+4) Install the library into your Maven repository:
+
+     $ mvn install
+
+5) If you do not use Maven to manage your own build, you can build a
+   .jar file to use:
+
+     $ mvn package
+
+   The .jar will be placed in the "target" directory.
+
+Installation - 'Lite' Version - With Maven
+==========================================
+
+Building the 'lite' version of the Java Protocol Buffers library is
+the same as building the full version, except that all commands are
+run using the 'lite' profile.  (see
+http://maven.apache.org/guides/introduction/introduction-to-profiles.html)
+
+E.g. to install the lite version of the jar, you would run:
+
+    $ mvn install -P lite
+
+The resulting artifact has the 'lite' classifier.  To reference it
+for dependency resolution, you would specify it as:
+
+```
+  <dependency>
+    <groupId>com.google.protobuf</groupId>
+    <artifactId>protobuf-java</artifactId>
+    <version>${version}</version>
+    <classifier>lite</classifier>
+  </dependency>
+```
+
+Installation - Without Maven
+============================
+
+If you would rather not install Maven to build the library, you may
+follow these instructions instead.  Note that these instructions skip
+running unit tests.
+
+1) Build the C++ code, or obtain a binary distribution of protoc.  If
+   you install a binary distribution, make sure that it is the same
+   version as this package.  If in doubt, run:
+
+     $ protoc --version
+
+   If you built the C++ code without installing, the compiler binary
+   should be located in ../src.
+
+2) Invoke protoc to build DescriptorProtos.java:
+
+     $ protoc --java_out=src/main/java -I../src \
+         ../src/google/protobuf/descriptor.proto
+
+3) Compile the code in src/main/java using whatever means you prefer.
+
+4) Install the classes wherever you prefer.
+
+Usage
+=====
+
+The complete documentation for Protocol Buffers is available via the
+web at:
+
+  https://developers.google.com/protocol-buffers/