Don't hard code protobuf specific file extensions in cpp_generator.cc

This file is shared with the Flatbuffers project as well, which does not
use "pb" file extensions.

src/compiler/config.h

@@ -96,4 +96,11 @@ typedef GRPC_CUSTOM_STRINGOUTPUTSTREAM StringOutputStream;
 }  // namespace protobuf
 }  // namespace grpc
 
+namespace grpc_cpp_generator {
+
+static const char * const kCppGeneratorMessageHeaderExt = ".pb.h";
+static const char * const kCppGeneratorServiceHeaderExt = ".grpc.pb.h";
+
+}  // namespace grpc_cpp_generator
+
 #endif  // SRC_COMPILER_CONFIG_H

src/compiler/cpp_generator.cc

@@ -40,9 +40,6 @@
 namespace grpc_cpp_generator {
 namespace {
 
-grpc::string message_header_ext() { return ".pb.h"; }
-grpc::string service_header_ext() { return ".grpc.pb.h"; }
-
 template <class T>
 grpc::string as_string(T x) {
   std::ostringstream out;
@@ -113,7 +110,7 @@ grpc::string GetHeaderPrologue(grpc_generator::File *file,
     vars["filename"] = file->filename();
     vars["filename_identifier"] = FilenameIdentifier(file->filename());
     vars["filename_base"] = file->filename_without_ext();
-    vars["message_header_ext"] = message_header_ext();
+    vars["message_header_ext"] = kCppGeneratorMessageHeaderExt;
 
     printer->Print(vars, "// Generated by the gRPC C++ plugin.\n");
     printer->Print(vars,
@@ -1039,8 +1036,8 @@ grpc::string GetSourcePrologue(grpc_generator::File *file,
 
     vars["filename"] = file->filename();
     vars["filename_base"] = file->filename_without_ext();
-    vars["message_header_ext"] = message_header_ext();
-    vars["service_header_ext"] = service_header_ext();
+    vars["message_header_ext"] = kCppGeneratorMessageHeaderExt;
+    vars["service_header_ext"] = kCppGeneratorServiceHeaderExt;
 
     printer->Print(vars, "// Generated by the gRPC C++ plugin.\n");
     printer->Print(vars,
@@ -1425,8 +1422,8 @@ grpc::string GetMockPrologue(grpc_generator::File *file,
 
     vars["filename"] = file->filename();
     vars["filename_base"] = file->filename_without_ext();
-    vars["message_header_ext"] = message_header_ext();
-    vars["service_header_ext"] = service_header_ext();
+    vars["message_header_ext"] = kCppGeneratorMessageHeaderExt;
+    vars["service_header_ext"] = kCppGeneratorServiceHeaderExt;
 
     printer->Print(vars, "// Generated by the gRPC C++ plugin.\n");
     printer->Print(vars,