Reduce absolute paths in generated ninja files

ui/vector_icons/vector_icons.gni

 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Creates an action that aggregates vector icon files (.icon) into a C++ file.
 # In addition to running the action, the outputs should be added to another
 # target's sources for compilation.
 #
 # Parameters
 #
@@ skipping 33 matching lines @@
 
     outputs = [
       output_cc,
       output_h,
     ]
 
     response_file_contents =
         rebase_path(invoker.icons, root_build_dir, invoker.icon_directory)
 
     args = [
-      "--working_directory=" + rebase_path(invoker.icon_directory),
+      "--working_directory=" +
+          rebase_path(invoker.icon_directory, root_build_dir),
       "--file_list={{response_file_name}}",
       "--output_cc=" + rebase_path(output_cc, root_build_dir),
       "--output_h=" + rebase_path(output_h, root_build_dir),
     ]
   }
 }