Handle content_shell failing to start on android as device going offline.

Tools/Scripts/webkitpy/layout_tests/port/android.py

 # Copyright (C) 2012 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 834 matching lines @@
         self._android_devices.set_device_prepared(self._android_commands.get_serial())
 
     def _log_error(self, message):
         _log.error('[%s] %s' % (self._android_commands.get_serial(), message))
 
     def _log_debug(self, message):
         if self._debug_logging:
             _log.debug('[%s] %s' % (self._android_commands.get_serial(), message))
 
     def _abort(self, message):
-        raise AssertionError('[%s] %s' % (self._android_commands.get_serial(), message))
+        raise driver.DeviceOffline('[%s] %s' % (self._android_commands.get_serial(), message))
 
     @staticmethod
     def _extract_hashes_from_md5sum_output(md5sum_output):
         assert md5sum_output
         return [line.split('  ')[0] for line in md5sum_output]
 
     def _files_match(self, host_file, device_file):
         assert self._port.host.filesystem.exists(host_file)
         device_hashes = self._extract_hashes_from_md5sum_output(
                 self._port.host.executive.popen(self._android_commands.adb_command() + ['shell', MD5SUM_DEVICE_PATH, device_file],
@@ skipping 272 matching lines @@
         return command
 
     def _read_prompt(self, deadline):
         last_char = ''
         while True:
             current_char = self._server_process.read_stdout(deadline, 1)
             if current_char == ' ':
                 if last_char in ('#', '$'):
                     return
             last_char = current_char