Bug 1585070 - move nsPIDOMWindowOuter::mIsActive to BrowsingContext. r=farre

A new BrowsingContext field, isActiveBrowserWindow, has been added
to track the active browser window for the :-moz-window-inactive
pseudoclass. This field takes the place of
nsPIDOMWindowOuter::mIsActive.

With this change :-moz-window-inactive is now fission compatible.

Differential Revision: https://phabricator.services.mozilla.com/D86422