Use $root:default as a "default" rule

tools/gn/build_settings.cc

Index: tools/gn/build_settings.cc
diff --git a/tools/gn/build_settings.cc b/tools/gn/build_settings.cc
index c60002de7ea95caff8266fe3b71f36b75f2b6410..4cf2b8679b0795e6cd938ad036cb4bfca243bb6a 100644
--- a/tools/gn/build_settings.cc
+++ b/tools/gn/build_settings.cc
@@ -25,6 +25,10 @@ BuildSettings::BuildSettings(const BuildSettings& other)
 BuildSettings::~BuildSettings() {
 }
 
+void BuildSettings::SetRootTargetLabel(const Label& r) {
+  root_target_label_ = r;
+}
+
 void BuildSettings::SetRootPath(const base::FilePath& r) {
   DCHECK(r.value()[r.value().size() - 1] != base::FilePath::kSeparators[0]);
   root_path_ = r.NormalizePathSeparatorsTo('/');