Add support for viewport size adjustments in wptrunner.

tests/wpt/harness/wptrunner/executors/executorselenium.py

@@ -247,7 +247,10 @@ class SeleniumRefTestExecutor(RefTestExecutor):
 
         return self.convert_result(test, result)
 
-    def screenshot(self, test):
+    def screenshot(self, test, viewport_size):
+        # https://github.com/w3c/wptrunner/issues/166
+        assert viewport_size is None
+
         return SeleniumRun(self._screenshot,
                            self.protocol.webdriver,
                            self.test_url(test),