third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/patches/0008-uninline_get_empty_string.patch

 unchanged:
 --- protobuf-cleaned/src/google/protobuf/generated_message_util.cc      2015-12-30 13:21:46.000000000 -0800
 +++ protobuf-patched/src/google/protobuf/generated_message_util.cc      2016-03-31 13:25:32.180017848 -0700
 @@ -60,6 +60,12 @@
    OnShutdown(&DeleteEmptyString);
  }
  
 +const ::std::string& GetEmptyString() {
 +  ::google::protobuf::GoogleOnceInit(&empty_string_once_init_, &InitEmptyString);
 +  return GetEmptyStringAlreadyInited();
 +}
 +
 +
  int StringSpaceUsedExcludingSelf(const string& str) {
    const void* start = &str;
    const void* end = &str + 1;
 only in patch2:
 unchanged:
 --- protobuf-cleaned/src/google/protobuf/generated_message_util.h       2015-12-30 13:21:46.000000000 -0800
 +++ protobuf-patched/src/google/protobuf/generated_message_util.h       2016-03-31 13:25:40.252007316 -0700
-@@ -84,10 +84,8 @@
-   assert(empty_string_ != NULL);
-   return *empty_string_;
+@@ -120,10 +120,8 @@
+   return fixed_address_empty_string.get();
  }
+ 
 -LIBPROTOBUF_EXPORT inline const ::std::string& GetEmptyString() {
 -  ::google::protobuf::GoogleOnceInit(&empty_string_once_init_, &InitEmptyString);
 -  return GetEmptyStringAlreadyInited();
 -}
 +
 +LIBPROTOBUF_EXPORT const ::std::string& GetEmptyString();
  
  LIBPROTOBUF_EXPORT int StringSpaceUsedExcludingSelf(const string& str);