Protobuf: Cherry-pick upstream r427.

third_party/protobuf/src/google/protobuf/descriptor.cc

Index: third_party/protobuf/src/google/protobuf/descriptor.cc
diff --git a/third_party/protobuf/src/google/protobuf/descriptor.cc b/third_party/protobuf/src/google/protobuf/descriptor.cc
index 58c7aa1ce7d632d894b8fc8067145e97ed390b96..5bdd565c156b78c94d3db5adf7acc1926fd5ee2b 100644
--- a/third_party/protobuf/src/google/protobuf/descriptor.cc
+++ b/third_party/protobuf/src/google/protobuf/descriptor.cc
@@ -2356,7 +2356,7 @@ Symbol DescriptorBuilder::LookupSymbolNoPlaceholder(
   //   }
   // So, we look for just "Foo" first, then look for "Bar.baz" within it if
   // found.
-  int name_dot_pos = name.find_first_of('.');
+  string::size_type name_dot_pos = name.find_first_of('.');
   string first_part_of_name;
   if (name_dot_pos == string::npos) {
     first_part_of_name = name;