GN: get echo-headers sources via exec_script

gyp/gputest.gyp

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'targets': [
     {
       'target_name': 'skgputest',
       'product_name': 'skia_skgputest',
       'type': 'static_library',
@@ skipping 11 matching lines @@
       ],
       'all_dependent_settings': {
         'include_dirs': [
           '../tools/gpu',
         ],
       },
       'dependencies': [
         'skia_lib.gyp:skia_lib',
       ],
       'sources': [
-        '<!@(python find.py ../tools/gpu "*")'
+        '<!@(python find.py "*" ../tools/gpu)'
       ],
       'conditions': [
         [ 'skia_mesa', { 'dependencies': [ 'osmesa' ] } ],
         [ 'skia_angle', {
           'dependencies': [
             'angle.gyp:*',
           ],
           'export_dependent_settings': [
             'angle.gyp:*',
           ],
@@ skipping 70 matching lines @@
         'conditions': [
           [ 'skia_os == "mac"', {
             'link_settings': { 'library_dirs' : [ '/opt/X11/lib' ], },
             'include_dirs': [ '/opt/X11/include', ],
           }],
         ],
       },
     },
   ],
 }