Merge branch 'fix-web-ide-header-button-background' into 'master'

Fix ide-tree-header button background

See merge request gitlab-org/gitlab-ce!20662

app/assets/javascripts/ide/components/new_dropdown/button.vue

@@ -38,6 +38,7 @@ export default {
   <button
     :aria-label="label"
     type="button"
+    class="btn-blank"
     @click.stop.prevent="clicked"
   >
     <icon