Update //third_party/protobuf to version 3.

third_party/protobuf/patches/0006-fix-clang-type-errors.patch

Index: third_party/protobuf/patches/0006-fix-clang-type-errors.patch
diff --git a/third_party/protobuf/patches/0006-fix-clang-type-errors.patch b/third_party/protobuf/patches/0006-fix-clang-type-errors.patch
new file mode 100644
index 0000000000000000000000000000000000000000..481ecb86becdb44fab35c7d9ff9056db1a543187
--- /dev/null
+++ b/third_party/protobuf/patches/0006-fix-clang-type-errors.patch
@@ -0,0 +1,21 @@
+--- protobuf-cleaned/src/google/protobuf/map.h	2015-12-30 13:21:46.000000000 -0800
++++ protobuf-patched/src/google/protobuf/map.h	2016-03-31 13:25:36.968011603 -0700
+@@ -545,10 +545,16 @@
+     }
+ 
+ #if __cplusplus >= 201103L && !defined(GOOGLE_PROTOBUF_OS_APPLE) && \
+-    !defined(GOOGLE_PROTOBUF_OS_NACL) && !defined(GOOGLE_PROTOBUF_OS_ANDROID)
++    !defined(GOOGLE_PROTOBUF_OS_NACL) &&                            \
++    !defined(GOOGLE_PROTOBUF_OS_ANDROID) &&                         \
++    !defined(GOOGLE_PROTOBUF_OS_EMSCRIPTEN)
+     template<class NodeType, class... Args>
+     void construct(NodeType* p, Args&&... args) {
+-      new (static_cast<void*>(p)) NodeType(std::forward<Args>(args)...);
++      // Clang 3.6 doesn't compile static casting to void* directly. (Issue #1266)
++      // According C++ standard 5.2.9/1: "The static_cast operator shall not cast
++      // away constness". So first the maybe const pointer is casted to const void* and
++      // after the const void* is const casted.
++      new (const_cast<void*>(static_cast<const void*>(p))) NodeType(std::forward<Args>(args)...);
+     }
+ 
+     template<class NodeType>