[sources] Fix incorrect styling of expandable containers.

https://crrev.com/c/6189464 moved the `viewContainerStyles` onto the
wrong widget (`ContainerWidget` vs.  `ExpandableContainerWidget`).

Fixed: 391829280
Change-Id: Ibc1df84556fbc20837694d1f8b2ee3cac00bb89f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6194055
Commit-Queue: Benedikt Meurer <[email protected]>
Reviewed-by: Kateryna Prokopenko <[email protected]>
Auto-Submit: Benedikt Meurer <[email protected]>
Commit-Queue: Kateryna Prokopenko <[email protected]>

Author: bmeurer

front_end/ui/legacy/ViewManager.ts

@@ -403,8 +403,6 @@ export class ContainerWidget extends VBox {
   }
 
   override wasShown(): void {
-    super.wasShown();
-    this.registerCSSFiles([viewContainersStyles]);
     void this.materialize().then(() => {
       const widget = widgetForView.get(this.view);
       if (widget) {
@@ -454,6 +452,8 @@ class ExpandableContainerWidget extends VBox {
   }
 
   override wasShown(): void {
+    super.wasShown();
+    this.registerCSSFiles([viewContainersStyles]);
     if (this.widget && this.materializePromise) {
       void this.materializePromise.then(() => {
         if (this.titleElement.classList.contains('expanded') && this.widget) {