extensions: fix dev controls spacing issue.

R=kalman@chromium.org
BUG=455196
TEST=visual

Review URL: https://codereview.chromium.org/900993002

Cr-Commit-Position: refs/heads/master@{#314658}

chrome/browser/resources/extensions/extensions.css

@@ -49,6 +49,10 @@ html:not(.focus-outline-visible)
   white-space: nowrap;
 }
 
+#dev-controls .button-container button:not(:last-of-type) {
+  -webkit-margin-end: 5px;
+}
+
 #apps-developer-tools-promo {
   align-items: center;
   border-top: 1px solid #eee;