gn: Support build directories outside the source tree.

tools/gn/value_extractors.cc

Index: tools/gn/value_extractors.cc
diff --git a/tools/gn/value_extractors.cc b/tools/gn/value_extractors.cc
index 9e6a37edc95c6c27478b0434ddc7f49d6ad9d645..6dfaa2f677e35ff5c315896826e292dc2fdae885 100644
--- a/tools/gn/value_extractors.cc
+++ b/tools/gn/value_extractors.cc
@@ -58,8 +58,6 @@ bool ListValueUniqueExtractor(const Value& value,
   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)
@@ -71,13 +69,6 @@ struct RelativeFileConverter {
       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;