Fix BenchmarkRunner to only import selenium when needed

Fix BenchmarkRunner to only import selenium when needed
https://bugs.webkit.org/show_bug.cgi?id=174808

Patch by Matthew Stewart <matthew_r_stewart@apple.com> on 2017-07-24
Reviewed by Stephanie Lewis.

* Scripts/webkitpy/benchmark_runner/browser_driver/linux_chrome_driver.py:
(LinuxChromeDriver.launch_driver):
* Scripts/webkitpy/benchmark_runner/browser_driver/linux_firefox_driver.py:
(LinuxFirefoxDriver.launch_driver):
* Scripts/webkitpy/benchmark_runner/browser_driver/osx_chrome_driver.py:
(OSXChromeDriver.launch_driver):
(OSXChromeCanaryDriver.launch_driver):
(create_chrome_options):
* Scripts/webkitpy/benchmark_runner/browser_driver/osx_firefox_driver.py:
(OSXFirefoxDriver.launch_driver):
(OSXFirefoxNightlyDriver.launch_driver):
* Scripts/webkitpy/benchmark_runner/browser_driver/osx_safari_driver.py:
(OSXSafariDriver.launch_driver):
* Scripts/webkitpy/benchmark_runner/webdriver_benchmark_runner.py:
(WebDriverBenchmarkRunner._run_one_test):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@219860 268f45cc-cd09-0410-ab3c-d52691b4dbfc