tbb-testsuite: fix the download_pdf test

The download_pdf test currently fails with the error:

Traceback (most recent call last):
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_harness-4.3.0-py2.7.egg/marionette_harness/marionette_test/testcases.py", line 156, in run
testMethod()
File "/home/tbb-testsuite/tbb-testsuite/marionette/tor_browser_tests/test_download_pdf.py", line 24, in test_download_pdf
m.set_window_size(1024, 300)
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 2063, in set_window_size
return self._send_message("setWindowSize", body)
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/decorators.py", line 23, in _
return func(*args, **kwargs)
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 740, in _send_message
    self._handle_error(err)
  File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 764, in _handle_error
    raise errors.lookup(error)(message, stacktrace=stacktrace)
UnknownCommandException: setWindowSize
stacktrace:
	WebDriverError@chrome://marionette/content/error.js:178:5
	UnknownCommandError@chrome://marionette/content/error.js:518:5
despatch@chrome://marionette/content/server.js:286:13
execute@chrome://marionette/content/server.js:267:11
onPacket/<@chrome://marionette/content/server.js:242:15
onPacket@chrome://marionette/content/server.js:241:8
_onJSONObjectReady/<@chrome://marionette/content/transport.js:500:9