Update //third_party/protobuf to version 3.

third_party/protobuf/patches/0009-uninline-arenastring.patch

+unchanged:
+--- protobuf-cleaned/src/google/protobuf/arenastring.cc 2015-12-30 13:21:46.000000000 -0800
++++ protobuf-patched/src/google/protobuf/arenastring.cc 2016-03-31 13:25:40.888006485 -0700
+@@ -48,6 +48,23 @@
+   }
+ }
+ 
++::std::string* ArenaStringPtr::MutableNoArena(
++     const ::std::string* default_value) {
++  if (ptr_ == default_value) {
++    CreateInstanceNoArena(default_value);
++  }
++  return ptr_;
++}
++
++
++void ArenaStringPtr::DestroyNoArena(const ::std::string* default_value) {
++  if (ptr_ != default_value) {
++    delete ptr_;
++  }
++  ptr_ = NULL;
++}
++
++
+ }  // namespace internal
+ }  // namespace protobuf
+ }  // namespace google
+only in patch2:
+unchanged:
+--- protobuf-cleaned/src/google/protobuf/arenastring.h  2015-12-30 13:21:46.000000000 -0800
++++ protobuf-patched/src/google/protobuf/arenastring.h  2016-03-31 13:25:40.888006485 -0700
+@@ -219,12 +219,7 @@
+     return *ptr_;
+   }
+ 
+-  inline ::std::string* MutableNoArena(const ::std::string* default_value) {
+-    if (ptr_ == default_value) {
+-      CreateInstanceNoArena(default_value);
+-    }
+-    return ptr_;
+-  }
++  ::std::string* MutableNoArena(const ::std::string* default_value);
+ 
+   inline ::std::string* ReleaseNoArena(const ::std::string* default_value) {
+     if (ptr_ == default_value) {
+@@ -248,12 +243,7 @@
+     }
+   }
+ 
+-  inline void DestroyNoArena(const ::std::string* default_value) {
+-    if (ptr_ != default_value) {
+-      delete ptr_;
+-    }
+-    ptr_ = NULL;
+-  }
++  void DestroyNoArena(const ::std::string* default_value);
+ 
+   inline void ClearToEmptyNoArena(const ::std::string* default_value) {
+     if (ptr_ == default_value) {