fix(checkbox): Prevent checkboxes and radios from changing size with multiline label (#497)

Author: Mike Yu

packages/mdc-checkbox/mdc-checkbox.scss

@@ -52,6 +52,7 @@
   box-sizing: content-box;
   width: $mdc-checkbox-size;
   height: $mdc-checkbox-size;
+  flex: 0 0 $mdc-checkbox-size;
   padding: ($mdc-checkbox-touch-area - $mdc-checkbox-size) / 2;
   line-height: 0;
   white-space: nowrap;

packages/mdc-radio/mdc-radio.scss

@@ -42,6 +42,7 @@ $mdc-radio-transition-duration: 120ms;
   box-sizing: border-box;
   width: $mdc-radio-touch-area;
   height: $mdc-radio-touch-area;
+  flex: 0 0 $mdc-radio-touch-area;
   padding: ($mdc-radio-touch-area - $mdc-radio-ui-size) / 2;
   cursor: pointer;
   will-change: opacity, transform, border-color, background-color, color;