The CoreTextLayout class is clearly based on the ATSLayout class, so
in order to make it easier to compare code of the two, rename fields
back as they are in the latter. Also add the corresponding comments
where applicable.

Once the CoreText code fully works and has superseded the ATSUI code,
sure, we can change field naming convention if we want.

Change-Id: I0d47ad0206fa895339506471bb638bd04f426490