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..1bb3932ab32d233138e1c6790efae5efd6ebea4e 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.").
@@ -553,7 +553,8 @@ bool Setup::FillOtherConfig(const CommandLine& cmdline) {
     }
 
     Label root_target_label =
-        Label::Resolve(SourceDir("//"), Label(), *root_value, &err);
+        Label::Resolve(SourceDir("//"), Label(), *root_value,
+                       build_settings_.root_path(), &err);
     if (err.has_error()) {
       err.PrintToStdout();
       return false;