gn: Support build directories outside the source tree.

tools/gn/value_extractors.cc

 // Copyright (c) 2013 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.
 
 #include "tools/gn/value_extractors.h"
 
 #include "tools/gn/build_settings.h"
 #include "tools/gn/err.h"
 #include "tools/gn/label.h"
 #include "tools/gn/source_dir.h"
@@ skipping 40 matching lines @@
       *err = Err(item, "Duplicate item in list");
       size_t previous_index = dest->IndexOf(new_one);
       err->AppendSubErr(Err(input_list[previous_index],
                             "This was the previous definition."));
       return false;
     }
   }
   return true;
 }
 
-// This extractor rejects files with system-absolute file paths. If we need
-// that in the future, we'll have to add some flag to control this.
 struct RelativeFileConverter {
   RelativeFileConverter(const BuildSettings* build_settings_in,
                         const SourceDir& current_dir_in)
       : build_settings(build_settings_in),
         current_dir(current_dir_in) {
   }
   bool operator()(const Value& v, SourceFile* out, Err* err) const {
     if (!v.VerifyTypeIs(Value::STRING, err))
       return false;
     *out = current_dir.ResolveRelativeFile(v.string_value(),
                                            build_settings->root_path_utf8());
-    if (out->is_system_absolute()) {
-      *err = Err(v, "System-absolute file path.",
-          "You can't list a system-absolute file path here. Please include "
-          "only files in\nthe source tree. Maybe you meant to begin with two "
-          "slashes to indicate an\nabsolute path in the source tree?");
-      return false;
-    }
     return true;
   }
   const BuildSettings* build_settings;
   const SourceDir& current_dir;
 };
 
 struct RelativeDirConverter {
   RelativeDirConverter(const BuildSettings* build_settings_in,
                        const SourceDir& current_dir_in)
       : build_settings(build_settings_in),
@@ skipping 119 matching lines @@
 }
 
 bool ExtractRelativeFile(const BuildSettings* build_settings,
                          const Value& value,
                          const SourceDir& current_dir,
                          SourceFile* file,
                          Err* err) {
   RelativeFileConverter converter(build_settings, current_dir);
   return converter(value, file, err);
 }