… pending frames.

Also change when pipelines become active.

This makes the constellation activate a pipeline as the current frame
when it is ready to do initial reflow, rather than when it is ready
to paint.

This fixes a number of intermittent failures that could previously occur
if an iframe was not visible - which would mean it was never moved from
a pending frame in the constellation to an active frame.

(It happens that webrender exposes these intermittents as permanent failures).