Make run-benchmark script supports 'config' key in test plan.

Make run-benchmark script supports 'config' key in test plan.
https://bugs.webkit.org/show_bug.cgi?id=167715

Reviewed by Ryosuke Niwa.

Add support for 'config' key in test paln.
'config' key is a dictionary that specifies environment configurations while test is running.
Specifies orientation to be portrait in MotionMark benchmark.

* Scripts/webkitpy/benchmark_runner/README.md:
* Scripts/webkitpy/benchmark_runner/benchmark_runner.py:
(BenchmarkRunner.__init__):
(BenchmarkRunner._run_benchmark):
* Scripts/webkitpy/benchmark_runner/browser_driver/browser_driver.py:
(BrowserDriver.prepare_env):
* Scripts/webkitpy/benchmark_runner/browser_driver/linux_browser_driver.py:
(LinuxBrowserDriver.prepare_env):
* Scripts/webkitpy/benchmark_runner/browser_driver/osx_browser_driver.py:
(OSXBrowserDriver.prepare_env):
* Scripts/webkitpy/benchmark_runner/browser_driver/osx_safari_driver.py:
(OSXSafariDriver.prepare_env):
* Scripts/webkitpy/benchmark_runner/data/plans/motionmark.plan:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@211549 268f45cc-cd09-0410-ab3c-d52691b4dbfc