Send proper MIME type along with PPAPI test CSS files.

ppapi/ppapi_tests.gypi

 # Copyright (c) 2012 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.
 
 {
   'targets': [
     {
       # GN version: //ppapi:ppapi_tests
       'target_name': 'ppapi_tests',
       'type': 'loadable_module',
@@ skipping 12 matching lines @@
         'ppapi_internal.gyp:ppapi_shared',
       ],
       'copies': [
         {
           'destination': '<(PRODUCT_DIR)',
           'files': [
             # Keep 'test_case.html.mock-http-headers' with 'test_case.html'.
             'tests/test_case.html',
             'tests/test_case.html.mock-http-headers',
             'tests/test_page.css',
+            'tests/test_page.css.mock-http-headers',
             'tests/ppapi_nacl_tests_newlib.nmf',
           ],
         },
         {
           'destination': '<(PRODUCT_DIR)/test_url_loader_data',
           'files': [
             'tests/test_url_loader_data/hello.txt',
           ],
         },
       ],
@@ skipping 603 matching lines @@
       'sources': [
         'examples/compositor/compositor.cc',
         'examples/compositor/spinning_cube.cc',
         'examples/compositor/spinning_cube.h',
       ],
     },
     # Adding a new PPAPI example? Don't forget to update the GN build.
     # See //ppapi/examples/BUILD.gn
   ],
 }