gn: Include source files outside the source root for Xcode workspace

tools/gn/xcode_writer.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_GN_XCODE_WRITER_H_
 #define TOOLS_GN_XCODE_WRITER_H_
 
 #include <iosfwd>
 #include <map>
 #include <memory>
@@ skipping 64 matching lines @@
                              const std::string& ninja_extra_args,
                              const BuildSettings* build_settings,
                              TargetOsType target_os);
 
   // Generates the "sources.xcodeproj" project that reference all source
   // files to allow Xcode to index them.
   void CreateSourcesProject(const std::vector<const Target*>& targets,
                             const SourceDir& root_build_dir,
                             const PBXAttributes& attributes,
                             const std::string& source_path,
+                            const std::string& absolute_source_path,
                             const std::string& config_name,
                             TargetOsType target_os);
 
   bool WriteFiles(const BuildSettings* build_settings, Err* err);
   bool WriteProjectFile(const BuildSettings* build_settings,
                         PBXProject* project,
                         Err* err);
 
   void WriteWorkspaceContent(std::ostream& out);
   void WriteProjectContent(std::ostream& out, PBXProject* project);
 
   std::string name_;
   std::vector<std::unique_ptr<PBXProject>> projects_;
 
   DISALLOW_COPY_AND_ASSIGN(XcodeWriter);
 };
 
 #endif  // TOOLS_GN_XCODE_WRITER_H_