fix(material-experimental/mdc-button): Allow typography mixin to consume 2018 style configs

See angular#21059 for context.

Author: mmalerba

src/material-experimental/mdc-button/_button-theme.scss

@@ -167,6 +167,7 @@ $mat-button-state-target: '.mdc-button__ripple';
 }
 
 @mixin mat-mdc-button-typography($config-or-theme) {
+  $config: mat-private-typography-normalized-config(mat-get-typography-config($config-or-theme));
   $config: mat-get-typography-config($config-or-theme);
   @include mat-using-mdc-typography($config) {
     @include mdc-button-without-ripple($query: $mat-typography-styles-query);
@@ -253,6 +254,7 @@ $mat-button-state-target: '.mdc-button__ripple';
 }
 
 @mixin mat-mdc-fab-typography($config-or-theme) {
+  $config: mat-private-typography-normalized-config(mat-get-typography-config($config-or-theme));
   $config: mat-get-typography-config($config-or-theme);
   @include mat-using-mdc-typography($config) {
     @include mdc-fab-without-ripple($query: $mat-typography-styles-query);
@@ -326,6 +328,7 @@ $mat-button-state-target: '.mdc-button__ripple';
 }
 
 @mixin mat-mdc-icon-button-typography($config-or-theme) {
+  $config: mat-private-typography-normalized-config(mat-get-typography-config($config-or-theme));
   $config: mat-get-typography-config($config-or-theme);
   @include mat-using-mdc-typography($config) {
     @include mdc-icon-button-without-ripple($query: $mat-typography-styles-query);