Simplify handling of {ib} splits. Bug 390425, r+sr=roc, a=dbaron

layout/base/nsCSSFrameConstructor.cpp

layout/base/nsCSSFrameConstructor.h

layout/base/nsPresShell.cpp

layout/reftests/ib-split/insert-into-split-inline-1-ref.html

layout/reftests/ib-split/insert-into-split-inline-1a.html

layout/reftests/ib-split/insert-into-split-inline-1b.html

layout/reftests/ib-split/insert-into-split-inline-1c.html

layout/reftests/ib-split/insert-into-split-inline-1d.html

layout/reftests/ib-split/insert-into-split-inline-1e.html

layout/reftests/ib-split/insert-into-split-inline-1f.html

layout/reftests/ib-split/insert-into-split-inline-1g.html

layout/reftests/ib-split/insert-into-split-inline-1h.html

layout/reftests/ib-split/insert-into-split-inline-1i.html

layout/reftests/ib-split/insert-into-split-inline-1j.html

layout/reftests/ib-split/insert-into-split-inline-1k.html

layout/reftests/ib-split/insert-into-split-inline-1l.html

layout/reftests/ib-split/insert-into-split-inline-2-ref.html

layout/reftests/ib-split/insert-into-split-inline-2a.html

layout/reftests/ib-split/insert-into-split-inline-2b.html

layout/reftests/ib-split/insert-into-split-inline-2c.html