Convert sandbox_linux_unittests to run exclusively on Swarming

sandbox/sandbox_linux_unittests.isolate

 # 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.
 
 # Because of a limitation in isolate_driver.py, this file needs to be in
 # the same directory as the main .gyp file.
 
 {
   'conditions': [
     ['OS=="android" or OS=="linux"', {
       'variables': {
         'command': [
           '<(PRODUCT_DIR)/sandbox_linux_unittests',
         ],
         'files': [
           '<(PRODUCT_DIR)/sandbox_linux_unittests',
         ],
-        'read_only': 1,
       },
     }],
   ],
   'includes': [
     # This is needed because of base/ dependencies on
     # icudtl.dat.
     '../base/base.isolate',
   ],
 }