Reduce absolute paths in generated ninja files

mojo/public/js/BUILD.gn

 # Copyright 2014 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.
 
 interfaces_bindings_gen_dir = "$root_gen_dir/mojo/public/interfaces/bindings"
 
 source_set("js") {
   sources = [
     "constants.cc",
     "constants.h",
@@ skipping 59 matching lines @@
 
   # TODO(yzshen): Eventually we would like to use Closure Compiler to minify the
   # bindings instead of simply concatenating the files.
   script = "//v8/tools/concatenate-files.py"
 
   sources = new_bindings_js_files
   outputs = [
     compiled_file,
   ]
 
-  args = rebase_path(new_bindings_js_files)
-  args += [ rebase_path(compiled_file) ]
+  args = rebase_path(new_bindings_js_files, root_build_dir)
+  args += [ rebase_path(compiled_file, root_build_dir) ]
 
   deps = [
     "//mojo/public/interfaces/bindings:new_bindings__generator",
   ]
 }
 
 group("tests") {
   testonly = true
 
   data = [
     "tests/core_unittest.js",
   ]
 
   public_deps = [
     ":bindings",
   ]
 }