Bug 175126

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 never 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 on any of the methods they implement, but not when just loading/importing the class. So, yo avoid raising exceptions on another platforms we load the python modules that are platform specific on the functions rather than on the main file or the main class body. Example: https://trac.webkit.org/browser/trunk/Tools/Scripts/webkitpy/benchmark_runner/browser_driver/linux_browser_driver.py?rev=216213#L110 On r219850 <https://trac.webkit.org/r219850> a method for _screen_size() was added also for the OSX platform, but this method is called from the main file so its evaluated when the load of the classes is done to get the lists of platforms an browsers. Therefore it aborts on Linux like this: https://build.webkit.org/builders/GTK%20Linux%2064-bit%20Release%20%28Perf%29/builds/850/steps/benchmark-test/logs/stdio