Bug 1111360 - Remove the unused NS_IS_REFLOW_ERROR macro and an obsolete comment.  r=roc
Bug 447660 part 1 - Replace the #define DISABLE_FLOAT_BREAKING_IN_COLUMNS with a pref to enable fragmenting of floats inside columns.  Set the pref enabled by default in non-RELEASE builds only.  r=roc
Bug 447660 part 2 - Mark an existing next-in-flow of a float that is split due to it being NOT_COMPLETE as NOT being an overflow container.  r=roc
Bug 447660 part 3 - When collecting / pushing a float, make sure to also pick up its next-in-flows that are in same block.  r=roc

Move the AssertNoDuplicateContinuations in DrainPushedFloats earlier because
DrainOverflowLines() sometimes picks up overflow line with a placeholder for
a float that has a next-in-flow in this block and unmark those floats as
pushed floats because they aren't anymore.

Change AppendPushedFloat to push any next-in-flows that are in the same block
as the given float too.

Make DoCollectFloats collect next-in-flows in the same block.

Make VerifyOverflowSituation check some float list invariants too.
Bug 1111292 - Include the column gap(s) in the minimum width calculation when the number of columns is non-auto.  r=roc