Use $root:default as a "default" rule

tools/gn/build_settings.h

Index: tools/gn/build_settings.h
diff --git a/tools/gn/build_settings.h b/tools/gn/build_settings.h
index 3f9f45b9882acb0f470e871fa1fafea5a63c8821..3f41ae3a8de9943f724f3bbeea82f5e913c6dcdc 100644
--- a/tools/gn/build_settings.h
+++ b/tools/gn/build_settings.h
@@ -14,6 +14,7 @@
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "tools/gn/args.h"
+#include "tools/gn/label.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/source_dir.h"
 #include "tools/gn/source_file.h"
@@ -31,6 +32,10 @@ class BuildSettings {
   BuildSettings(const BuildSettings& other);
   ~BuildSettings();
 
+  // Root target label.
+  const Label& root_target_label() const { return root_target_label_; }
+  void SetRootTargetLabel(const Label& r);
+
   // Absolute path of the source root on the local system. Everything is
   // relative to this. Does not end in a [back]slash.
   const base::FilePath& root_path() const { return root_path_; }
@@ -104,6 +109,7 @@ class BuildSettings {
   }
 
  private:
+  Label root_target_label_;
   base::FilePath root_path_;
   std::string root_path_utf8_;
   base::FilePath secondary_source_path_;