[CrOS Network] Remove unnecessary scroll bar from network exclusions.

No margin was provided, so <cr-icon-button> used the default margin,
which happened to be -8px. This CL sets the margin to 0.

Bug: 981318
Change-Id: I6fa10935feecdda9626d3698ca10b707d28761e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825141
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700304}

ui/webui/resources/cr_components/chromeos/network/network_proxy_exclusions.html

@@ -16,6 +16,10 @@
         overflow-y: auto;
         padding: 5px;
       }
+
+      cr-icon-button {
+        --cr-icon-button-margin-end: 0;
+      }
     </style>
     <div id="container">
       <template is="dom-repeat" items="[[exclusions]]">