gn: Support build directories outside the source tree.

tools/gn/function_get_label_info.cc

Index: tools/gn/function_get_label_info.cc
diff --git a/tools/gn/function_get_label_info.cc b/tools/gn/function_get_label_info.cc
index c097941e278ca3c9849d8d826629ce6f8d1a4605..6c47abc336b07ec1b3a17be474b509ac1790ebd8 100644
--- a/tools/gn/function_get_label_info.cc
+++ b/tools/gn/function_get_label_info.cc
@@ -92,7 +92,9 @@ Value RunGetLabelInfo(Scope* scope,
 
   // Resolve the requested label.
   Label label = Label::Resolve(scope->GetSourceDir(),
-                               ToolchainLabelForScope(scope), args[0], err);
+                               ToolchainLabelForScope(scope), args[0],
+                               scope->settings()->build_settings()->root_path(),
+                               err);
   if (label.is_null())
     return Value();