refactor(material/chips): replace px with _trailing-icon-size variable

Updates previous Angular Components Chip component fix and makes it
adaptable by swapping the hard-coded 18px offset to equal the
variable for -icon-size.

Author: essjay05

src/material/chips/chip.scss

@@ -695,7 +695,7 @@ $_avatar-trailing-padding: 8px;
 
   // Used as a state overlay.
   &::after {
-    $offset: 18px; // size of the remove icon
+    $offset: $_trailing-icon-size; // size of the remove icon
     content: '';
     display: block;
     opacity: 0;