Replace --padding with --gr-button-padding

See change 320035.

Change-Id: I27532700d491ad28ccf26cbde19eff61290e0c58
diff --git a/ui/suggest-owners-trigger.js b/ui/suggest-owners-trigger.js
index 427fef6..3c7bf91 100644
--- a/ui/suggest-owners-trigger.js
+++ b/ui/suggest-owners-trigger.js
@@ -43,7 +43,7 @@
             text-decoration: none;
           }
           gr-button {
-            --padding: var(--spacing-xs) var(--spacing-s);
+            --gr-button-padding: var(--spacing-xs) var(--spacing-s);
           }
         </style>
         <gr-button