fix(list): set explicit box-sizing on icon

Sets an explicit `box-sizing` on the icons inside of lists, in order to avoid alignment issues if the consumer uses `border-box` globally.

Fixes angular#3863.

Author: crisbeto

src/lib/list/list.scss

@@ -86,6 +86,7 @@ $mat-dense-three-line-height: 76px;
   .mat-list-icon {
     width: $mat-list-icon-size;
     height: $mat-list-icon-size;
+    box-sizing: content-box;
     border-radius: 50%;
     padding: 4px;
   }