GitLab

The task runner being used in PreviewsOptimizationGuide was UI,
but it was named IO. The name has been updated to reflect the
actual task runner passed in.

Change-Id: Iab6e07c148ca087194d9b876dff4f4f5b285a153
Reviewed-on: https://chromium-review.googlesource.com/c/1338727Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Jered Gray <jegray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611251}