fix(button): hide overflow of buttons to prevent overflow of hover/ri…

…pple (angular#9424)

src/lib/button/_button-base.scss

@@ -34,6 +34,7 @@ $mat-mini-fab-padding: 8px !default;
 // Applies base styles to all button types.
 @mixin mat-button-base {
   box-sizing: border-box;
+  overflow: hidden;
   position: relative;
 
   // Reset browser <button> styles.