Toolbars that measure their own width call this and prefer its value over their measurement, so
stories and tests can pin a step in an environment with no layout engine. Components that only
read a step have no use for this — they call useShrinkStepValue instead.
Returns undefined|number
The overriding step, or undefined when nothing is overriding.
Reads a shrink step forced from outside, if any.
Toolbars that measure their own width call this and prefer its value over their measurement, so stories and tests can pin a step in an environment with no layout engine. Components that only read a step have no use for this — they call useShrinkStepValue instead.