Ensure marionette screenshots cover the viewport. (#223)

It turns out the default marionette screenshot mode will attempt to screenshot
the html element, including cropping below the viewport size. This isn't expected
behaviour for reftests, so disable it.

wptrunner/executors/executormarionette.py

@@ -508,7 +508,7 @@ def _screenshot(self, marionette, url, timeout):
 
         marionette.execute_async_script(self.wait_script)
 
-        screenshot = marionette.screenshot()
+        screenshot = marionette.screenshot(full=False)
         # strip off the data:img/png, part of the url
         if screenshot.startswith("data:image/png;base64,"):
             screenshot = screenshot.split(",", 1)[1]