Update //third_party/protobuf to version 3.

third_party/protobuf/benchmarks/readme.txt

+Contents
+--------
+
+This folder contains three kinds of file:
+
+- Code, such as ProtoBench.java, to build the benchmarking framework.
+- Protocol buffer definitions (.proto files)
+- Sample data files
+
+If we end up with a lot of different benchmarks it may be worth
+separating these out info different directories, but while there are
+so few they might as well all be together.
+
+Running a benchmark (Java)
+--------------------------
+
+1) Build protoc and the Java protocol buffer library. The examples
+   below assume a jar file (protobuf.jar) has been built and copied
+   into this directory.
+
+2) Build ProtoBench:
+   $ javac -d tmp -cp protobuf.jar ProtoBench.java
+   
+3) Generate code for the relevant benchmark protocol buffer, e.g.
+   $ protoc --java_out=tmp google_size.proto google_speed.proto
+   
+4) Build the generated code, e.g.
+   $ cd tmp
+   $ javac -d . -cp ../protobuf.jar benchmarks/*.java
+           
+5) Run the test. Arguments are given in pairs - the first argument
+   is the descriptor type; the second is the filename. For example:
+   $ java -cp .;../protobuf.jar com.google.protocolbuffers.ProtoBench
+          benchmarks.GoogleSize$SizeMessage1 ../google_message1.dat
+          benchmarks.GoogleSpeed$SpeedMessage1 ../google_message1.dat
+          benchmarks.GoogleSize$SizeMessage2 ../google_message2.dat
+          benchmarks.GoogleSpeed$SpeedMessage2 ../google_message2.dat
+          
+6) Wait! Each test runs for around 30 seconds, and there are 6 tests
+   per class/data combination. The above command would therefore take
+   about 12 minutes to run.
+
+   
+Benchmarks available
+--------------------
+
+From Google:
+google_size.proto and google_speed.proto, messages
+google_message1.dat and google_message2.dat. The proto files are
+equivalent, but optimized differently.