Add a useful error message for run-webkit-tests broken --target handling.

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py

Index: third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py b/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py
index e27855b767a9a0e2dd6d320b2387dd82b54a1fc3..ab51899fdfed89307ee3e1589cb3622d0a7f5464 100644
--- a/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py
@@ -60,6 +60,12 @@ def main(argv, stdout, stderr):
         host = Host()
 
     try:
+        # TODO: crbug.com/539509 - Drop this check and make --target work properly.
+        if options.configuration not in (None, 'Debug', 'Release', 'Debug_x64', 'Release_x64'):
+            raise NotImplementedError('--target must be either "Debug" or "Release"; other values do not work\n.'
+                                      'Use multiple //out directories for multiple builds if necessary.\n'
+                                      'See crbug.com/539509.')
+
         port = host.port_factory.get(options.platform, options)
     except NotImplementedError, e:
         # FIXME: is this the best way to handle unsupported port names?