Add orphaned tab check to WebKit as well.

This implements the orphaned editor fix for WebKit. Webkit wasn't
crashing before, but this causes webkit to show the same warning
webengine does if the editor is orhpaned (rather than silently
continuing). This allows the same BDD test to pass for both webkit and
webengine.

qutebrowser/browser/webkit/webkitelem.py

@@ -118,6 +118,8 @@ class WebKitElement(webelem.AbstractWebElement):
 
     def set_value(self, value):
         self._check_vanished()
+        if self._tab.is_deleted():
+            raise webelem.OrphanedError("Tab containing element vanished")
         if self.is_content_editable():
             log.webelem.debug("Filling {!r} via set_text.".format(self))
             self._elem.setPlainText(value)