Setup onclick handlers when attaching to DOM

When the connectedCallback is called on the panel item custom
element, set any button onclick handlers since they could have
been null'ed out if the panel item was removed from the DOM.

Bug: 1009318
Change-Id: I621bf1bcda85c877c1c52a182474e6ec294c63ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832188
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701450}

ui/file_manager/file_manager/foreground/elements/xf_panel_item.js

@@ -369,6 +369,16 @@ class PanelItem extends HTMLElement {
    */
   connectedCallback() {
     this.onclick = this.onClicked_.bind(this);
+
+    // Set click event handler references.
+    let button = this.shadowRoot.querySelector('#primary-action');
+    if (button) {
+      button.onclick = this.onclick;
+    }
+    button = this.shadowRoot.querySelector('#secondary-action');
+    if (button) {
+      button.onclick = this.onclick;
+    }
   }
 
   /**