gn: Support build directories outside the source tree.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index fdbf0f24798f4d417e203d8e0b4d094df05c3134..0757439849ea471aecc032578097d011d88f6fe2 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -441,7 +441,7 @@ bool Setup::FillSourceDir(const CommandLine& cmdline) {
 bool Setup::FillBuildDir(const std::string& build_dir, bool require_exists) {
   SourceDir resolved =
       SourceDirForCurrentDirectory(build_settings_.root_path()).
-          ResolveRelativeDir(build_dir);
+    ResolveRelativeDir(build_dir, build_settings_.root_path());
   if (resolved.is_null()) {
     Err(Location(), "Couldn't resolve build directory.",
         "The build directory supplied (\"" + build_dir + "\") was not valid.").