[Refactor Xcode Objects] Decouple file references and indexing target.

tools/gn/xcode_writer.cc

Index: tools/gn/xcode_writer.cc
diff --git a/tools/gn/xcode_writer.cc b/tools/gn/xcode_writer.cc
index 633a1c8db5ea35b354ff3bfb5297aa3c1628dae4..6f6b0d2bce66e7b94d4a711eb531de0477cfb372 100644
--- a/tools/gn/xcode_writer.cc
+++ b/tools/gn/xcode_writer.cc
@@ -353,7 +353,7 @@ void XcodeWriter::CreateSourcesProject(
   for (const SourceFile& source : sources) {
     std::string source_file =
         RebasePath(source.value(), source_dir, absolute_source_path);
-    sources_for_indexing->AddSourceFile(source_file);
+    sources_for_indexing->AddSourceFileToIndexingTarget(source_file);
   }
 
   projects_.push_back(std::move(sources_for_indexing));