nrwt: make --chromium work like --qt

nrwt: make --chromium work like --qt
https://bugs.webkit.org/show_bug.cgi?id=76875

Reviewed by Adam Barth.

--chromium used to have to be handled differently from --qt
due to the way the PortFactory was implemented; there's not
really a good reason for that any more so this patch makes
things slightly more consistent and eliminates the
options.chromium flag (--chromium is now truly a synonym for
--platform chromium).

* Scripts/webkitpy/layout_tests/port/factory.py:
(PortFactory._default_port):
(PortFactory.get):
* Scripts/webkitpy/layout_tests/port/factory_unittest.py:
(FactoryTest.setUp):
(FactoryTest.test_chromium_mac):
(FactoryTest.test_chromium_linux):
(FactoryTest.test_chromium_win):
* Scripts/webkitpy/layout_tests/run_webkit_tests.py:
(parse_args):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105674 268f45cc-cd09-0410-ab3c-d52691b4dbfc