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

See change 320035.

Change-Id: I74d63b4e35249184f8b4306e68c1b392897f48a2

web/gr-repo-chip.ts

diff --git a/web/gr-repo-chip.ts b/web/gr-repo-chip.ts
index 931f913..b0a361d 100644
--- a/web/gr-repo-chip.ts
+++ b/web/gr-repo-chip.ts
@@ -34,7 +34,7 @@
       display: inline-block;
     }
     gr-button {
-      --padding: 0;
+      --gr-button-padding: 0;
       vertical-align: top;
     }
     iron-icon {