third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/src/google/protobuf/testing/googletest.cc

Index: third_party/protobuf/src/google/protobuf/testing/googletest.cc
diff --git a/third_party/protobuf/src/google/protobuf/testing/googletest.cc b/third_party/protobuf/src/google/protobuf/testing/googletest.cc
index 2b9cddef57e68ca2e6a3a873893733b7f2caef6d..d45706b6548539b825903e6b8331ff195bd950ee 100644
--- a/third_party/protobuf/src/google/protobuf/testing/googletest.cc
+++ b/third_party/protobuf/src/google/protobuf/testing/googletest.cc
@@ -66,6 +66,9 @@ namespace protobuf {
 
 string TestSourceDir() {
 #ifndef GOOGLE_THIRD_PARTY_PROTOBUF
+#ifdef GOOGLE_PROTOBUF_TEST_SOURCE_PATH
+  return GOOGLE_PROTOBUF_TEST_SOURCE_PATH;
+#else
 #ifndef _MSC_VER
   // automake sets the "srcdir" environment variable.
   char* result = getenv("srcdir");
@@ -86,6 +89,7 @@ string TestSourceDir() {
     prefix += "/..";
   }
   return prefix + "/src";
+#endif  // GOOGLE_PROTOBUF_TEST_SOURCE_PATH
 #else
   return "third_party/protobuf/src";
 #endif  // GOOGLE_THIRD_PARTY_PROTOBUF