gn: Support build directories outside the source tree.

tools/gn/output_file.cc

Index: tools/gn/output_file.cc
diff --git a/tools/gn/output_file.cc b/tools/gn/output_file.cc
index d1fb88e8ee692951c9927856d8574c131743da1e..a9c7e63395c2c34b071e1f0e2574c22b211ef941 100644
--- a/tools/gn/output_file.cc
+++ b/tools/gn/output_file.cc
@@ -17,7 +17,8 @@ OutputFile::OutputFile(const base::StringPiece& str)
 OutputFile::OutputFile(const BuildSettings* build_settings,
                        const SourceFile& source_file)
     : value_(RebaseSourceAbsolutePath(source_file.value(),
-                                      build_settings->build_dir())) {
+                                      build_settings->build_dir(),
+                                      build_settings->root_path_utf8())) {
 }
 
 OutputFile::~OutputFile() {