grit: Use new --write-only-new flag.

build/grit_action.gypi

 # Copyright (c) 2011 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.
 
 # This file is meant to be included into an action to invoke grit in a
 # consistent manner. To use this the following variables need to be
 # defined:
 #   grit_grd_file: string: grd file path
 #   grit_out_dir: string: the output directory path
 
 # It would be really nice to do this with a rule instead of actions, but it
 # would need to determine inputs and outputs via grit_info on a per-file
 # basis. GYP rules don't currently support that. They could be extended to
 # do this, but then every generator would need to be updated to handle this.
 
 {
   'variables': {
     'grit_cmd': ['python', '<(DEPTH)/tools/grit/grit.py'],
     'grit_resource_ids%': '<(DEPTH)/tools/gritsettings/resource_ids',
     # This makes it possible to add more defines in specific targets,
     # instead of build/common.gypi .
     'grit_additional_defines%': [],
     'grit_rc_header_format%': [],
+
+    'conditions': [
+      # These scripts can skip writing generated files if they are identical
+      # to the already existing files, which avoids further build steps, like
+      # recompilation. However, a dependency (earlier build step) having a
+      # newer timestamp than an output (later build step) confuses some build
+      # systems, so only use this on ninja, which explicitly supports this use
+      # case (gyp turns all actions into ninja restat rules).
+      ['"<(GENERATOR)"=="ninja"', {
+        'write_only_new': '1',
+      }, {
+        'write_only_new': '0',
+      }],
+    ],
   },
   'inputs': [
     '<!@pymod_do_main(grit_info <@(grit_defines) <@(grit_additional_defines) '
         '--inputs <(grit_grd_file) -f "<(grit_resource_ids)")',
   ],
   'outputs': [
     '<!@pymod_do_main(grit_info <@(grit_defines) <@(grit_additional_defines) '
         '--outputs \'<(grit_out_dir)\' '
         '<(grit_grd_file) -f "<(grit_resource_ids)")',
   ],
   'action': ['<@(grit_cmd)',
              '-i', '<(grit_grd_file)', 'build',
              '-f', '<(grit_resource_ids)',
              '-o', '<(grit_out_dir)',
+             '--write-only-new=<(write_only_new)',
              '<@(grit_defines)',
              '<@(grit_additional_defines)',
              '<@(grit_rc_header_format)'],
   'message': 'Generating resources from <(grit_grd_file)',
 }