Make all the files mapped in when running unit_tests read only on Linux and OSX.

chrome/unit_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.
 {
   'conditions': [
     ['OS=="android" or OS=="linux"', {
       'variables': {
         'isolate_dependency_untracked': [
           '<(PRODUCT_DIR)/test_data/',
         ],
@@ skipping 55 matching lines @@
         'isolate_dependency_untracked': [
           '../net/tools/testserver/',
           '../third_party/pyftpdlib/',
           '../third_party/pywebsocket/',
           '../third_party/tlslite/',
           '../tools/swarming_client/',
           '<(PRODUCT_DIR)/pyproto/',
         ],
       },
     }],
+    ['OS=="linux" or OS=="mac"', {
+      'variables': {
+        # Can't be enabled on Windows due to http://crbug.com/342913
+        'read_only': 1,
+      },
+    }],
     ['OS=="mac"', {
       'variables': {
         'isolate_dependency_tracked': [
           '<(PRODUCT_DIR)/ffmpegsumo.so',
         ],
         'isolate_dependency_untracked': [
           '<(PRODUCT_DIR)/<(mac_product_name) Framework.framework/',
         ],
       },
     }],
@@ skipping 24 matching lines @@
           '../skia/ext/data/',
           '../ui/base/glib/',
         ],
       },
     }],
   ],
   'includes': [
     '../third_party/icu/icu.isolate',
   ],
 }