Followup eb95d7b0b2: don't hide the inspectors WindowSet at all

There's no point in hiding the WindowSet if we've already closed the
open window in it. In fact, there's no point in hiding a WindowSet
pretty much ever.

Change-Id: I49a02b5d255b266eb9e0a537cb64082eb0ad5e4b
Use visibility: hidden; for the ui.Context and its popup

They contain iframes and those need to not be display: none;
in order to make Firefox happy.

Bug: 57568
Change-Id: I177877bff3c3b18bfc87ee4e6afa7e60fea26ffc
Merge changes I177877bf,I49a02b5d

* changes:
  Use visibility: hidden; for the ui.Context and its popup
  Followup eb95d7b0b2: don't hide the inspectors WindowSet at all