Bug 1683188 - CrossProcessPaint code shouldn't mess with tab state from the content process. r=mattwoodrow,NeilDeakin, a=jcristau

There's JS running since we save the active status till we restore it,
so arbitrary things can happen, including receiving an IPC message from
the child saying that we're now really active.

If then we restore the wrong (old) status, stuff gets confused and
sadness ensues.

Screenshotting background tabs seems to work without this so it's not
clear to me why messing with the activeness state was necessary to begin
with.

Differential Revision: https://phabricator.services.mozilla.com/D101060