Do not add placeholder if page is already loaded.

Bug: 943038
Change-Id: I61cc89ad0539b5285f081d156da459ca93395f5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532726
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642979}
2 files changed