Bug 1865302 [Linux] Don't resume compositor if we're missing...

Bug 1865302 [Linux] Don't resume compositor if we're missing mCompositorWidgetDelegate. r=emilio, a=RyanVM

We pause compositor for scale changes to avoid flickering during repaint.
If compositor is deleted during the pause, don't try to resume it.
Just flip it back to enabled and let SetCompositorWidgetDelegate() or map event to resume it.

Differential Revision: https://phabricator.services.mozilla.com/D194110