Bug 1522637 - Part 8: Allow 'element_stale' to work after process swap, r=ato

The element has never existed in the new process, so it gets a missing
error rather than a stale error in that case.

Depends on D19692

Differential Revision: https://phabricator.services.mozilla.com/D19829

--HG--
extra : moz-landing-system : lando

testing/marionette/client/marionette_driver/expected.py

@@ -101,7 +101,9 @@ class element_stale(object):
             # Calling any method forces a staleness check
             self.el.is_enabled()
             return False
-        except errors.StaleElementException:
+        except (errors.StaleElementException, errors.NoSuchElementException):
+            # StaleElementException is raised when the element is gone, and
+            # NoSuchElementException is raised after a process swap.
             return True