Send proper MIME type along with PPAPI test CSS files.

ppapi/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.
 
 copy("copy_test_files") {
   visibility = [ ":*" ]
   sources = [
     # Keep "test_case.html.mock-http-headers" with "test_case.html".
     "tests/ppapi_nacl_tests_newlib.nmf",
     "tests/test_case.html",
     "tests/test_case.html.mock-http-headers",
     "tests/test_page.css",
+    "tests/test_page.css.mock-http-headers",
   ]
   outputs = [
     "$root_out_dir/{{source_file_part}}",
   ]
 }
 
 copy("copy_test_files2") {
   visibility = [ ":*" ]
   sources = [
     "tests/test_url_loader_data/hello.txt",
@@ skipping 107 matching lines @@
 
 executable("pepper_hash_for_uma") {
   sources = [
     "tools/pepper_hash_for_uma.cc",
   ]
 
   deps = [
     "//base",
   ]
 }