REGRESSION(r219850): run-benchmark script broken on Linux

REGRESSION(r219850): run-benchmark script broken on Linux
https://bugs.webkit.org/show_bug.cgi?id=175126

Reviewed by Stephanie Lewis.

The run-benchmark script dynamically generates the list of supported
browsers and platforms (currently Linux and OSX) by loading all
python files from Tools/Scripts/webkitpy/benchmark_runner/browser_driver
and getting the browser_name and platform variables from the
classes defined there.

This means that this classes should not raise an exception when
loaded on other platforms or otherwise they will broke the whole
script. Its fine if they raise an exception when executing any of
the methods they implement, but not when just loading/importing
the class.

Move the argument variable definitions that call on the platform
specific OSXBrowserDriver._screen_size() function from beeing
variables that are evaluated when loading the file, to be functions
that are only evaluated when the actual functionality needs to be
executed.

* Scripts/webkitpy/benchmark_runner/browser_driver/osx_chrome_driver.py:
(OSXChromeDriver.launch_url):
(OSXChromeCanaryDriver.launch_url):
(create_args):
(create_chrome_options):
(create_window_size_arg):
* Scripts/webkitpy/benchmark_runner/browser_driver/osx_firefox_driver.py:
(OSXFirefoxDriver.launch_url):
(OSXFirefoxNightlyDriver.launch_url):
(OSXFirefoxNightlyDriver.launch_driver):
(create_args):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220246 268f45cc-cd09-0410-ab3c-d52691b4dbfc