gn: Support build directories outside the source tree.

tools/gn/function_read_file.cc

Index: tools/gn/function_read_file.cc
diff --git a/tools/gn/function_read_file.cc b/tools/gn/function_read_file.cc
index f35e5a67fe18c25aaaec2b6d10e9b86ae65e8835..53f038dc7dcf2995ddf348ca072c122191b6b6c0 100644
--- a/tools/gn/function_read_file.cc
+++ b/tools/gn/function_read_file.cc
@@ -53,7 +53,8 @@ Value RunReadFile(Scope* scope,
 
   // Compute the file name.
   const SourceDir& cur_dir = scope->GetSourceDir();
-  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0].string_value());
+  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0].string_value(),
+      scope->settings()->build_settings()->root_path());
   base::FilePath file_path =
       scope->settings()->build_settings()->GetFullPath(source_file);