fix(mat-stepper): .mat-stepper-label should be a consistent size

[dpurp]

Provide a consistent font-size for .mat-stepper-label when body-1 and body-2 are different sizes.

With the Material Design (v1) typography, body-1 and body-2 are the same size, so this change is a noop; however this fixes a CSS bug in GMat when the sizes diverge.