[quickview] Address metrics TODO: record this UMA when idle

Since rendering work here is way more important, idle delay the UMA to
a better place: when the browser is idle.

Bug: 965370
Change-Id: Idf14450312b07a751901f3a3b0ea99f988890c5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662106Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669590}

ui/file_manager/file_manager/foreground/js/quick_view_controller.js

@@ -243,8 +243,9 @@ class QuickViewController {
     const entry = this.entries_[0];
     this.quickViewModel_.setSelectedEntry(entry);
 
-    // TODO(noel): Record UMA _after_ the rendering work below is done?
-    this.quickViewUma_.onEntryChanged(entry);
+    requestIdleCallback(() => {
+      this.quickViewUma_.onEntryChanged(entry);
+    });
 
     return Promise
         .all([