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

base/base_unittests.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.
 {
   'conditions': [
     ['OS=="android" or OS=="linux" or OS=="mac" or OS=="win"', {
       'variables': {
         'isolate_dependency_untracked': [
           'data/',
           'test/data/',
         ],
+        'read_only': 1,
       },
     }],
     ['OS=="linux"', {
       'variables': {
         'command': [
           '../testing/xvfb.py',
           '<(PRODUCT_DIR)',
           '<(PRODUCT_DIR)/base_unittests<(EXECUTABLE_SUFFIX)',
           '--brave-new-test-launcher',
           '--test-launcher-bot-mode',
@@ skipping 27 matching lines @@
     }],
     ['OS=="win"', {
       'variables': {
         'isolate_dependency_tracked': [
           '<(PRODUCT_DIR)/icudt.dll',
         ],
       },
     }],
   ],
 }