Make this.$.$iframe actually point to an iframe again

Window sets its this.$.$iframe to this.$element, but with
the recent-ish refactor that's now a div, not an iframe.
Set it to the actual iframe instead.

This broke the logic in TextInputMenuWidget that compensates
for the menu and its text input being in different frames.
This kept working by accident for non-modal windows because
their $element and $iframe have the same coordinates.

Change-Id: I46a64001152c51b41cd13c2b4100785d4ceb24a8
Tool: Use FlaggedElement mixin to let tools be flagged

Change-Id: Ic957222879b8ad8b430e9be6b257ecae7ed713ac
Merge "Make this.$.$iframe actually point to an iframe again"
Merge "Tool: Use FlaggedElement mixin to let tools be flagged"