Add extra content_shell per worker for virtual tests, except on Android.

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 461 matching lines @@
     def driver_stop_timeout(self):
         # The driver doesn't respond to closing stdin, so we might as well stop the driver immediately.
         return 0.0
 
     def default_child_processes(self):
         usable_devices = self._devices.usable_devices(self._executive)
         if not usable_devices:
             raise test_run_results.TestRunException(test_run_results.NO_DEVICES_EXIT_STATUS, "Unable to find any attached Android devices.")
         return len(usable_devices)
 
+    def max_drivers_per_process(self):
+        # Android falls over when we try to run multiple content_shells per worker.
+        # See https://codereview.chromium.org/1158323009/
+        return 1
+
     def check_wdiff(self, logging=True):
         return self._host_port.check_wdiff(logging)
 
     def check_build(self, needs_http, printer):
         exit_status = super(AndroidPort, self).check_build(needs_http, printer)
         if exit_status:
             return exit_status
 
         result = self._check_file_exists(self.path_to_md5sum(), 'md5sum utility')
         result = self._check_file_exists(self.path_to_md5sum_host(), 'md5sum host utility') and result
@@ skipping 789 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