Make all the files mapped in when running browser_tests read only.

chrome/browser_tests.isolate

 # 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.
 {
   'includes': [
     'chrome.isolate',
   ],
   'conditions': [
     ['OS=="linux"', {
       'variables': {
@@ skipping 66 matching lines @@
           '<(PRODUCT_DIR)/pnacl/',
           '<(PRODUCT_DIR)/pseudo_locales/',
           '<(PRODUCT_DIR)/pyproto/',
           '<(PRODUCT_DIR)/test_data/chrome/',
           '<(PRODUCT_DIR)/test_data/ppapi/',
           '<(PRODUCT_DIR)/test_url_loader_data/',
           'common/extensions/docs/examples/apps/calculator/tests/',
           'test/data/',
           'third_party/mock4js/',
         ],
+        'read_only': 1,
       },
     }],
     ['OS=="linux" or OS=="win"', {
       'variables': {
         'isolate_dependency_tracked': [
           '<(PRODUCT_DIR)/mock_nacl_gdb<(EXECUTABLE_SUFFIX)',
           '<(PRODUCT_DIR)/ppapi_nacl_tests_glibc_x64.nexe',
           '<(PRODUCT_DIR)/ppapi_nacl_tests_newlib_x64.nexe',
           '<(PRODUCT_DIR)/ppapi_nacl_tests_pnacl_newlib_x64.nexe',
         ],
@@ skipping 55 matching lines @@
     }],
     ['chromeos==1', {
       'variables': {
         'isolate_dependency_untracked': [
           '<(PRODUCT_DIR)/chromevox_test_data/',
         ],
       },
     }],
   ],
 }