WebUI: Add fallback for search engine favicons

This CL adds a default fallback of a favicon with an empty URL to
display the default globe icon, and also sends the search engine URL
as a fallback if the search engine favicon URL is not available.

Bug: 1002840
Change-Id: Ie1662e5b3c0e95418abc9bb4bf9a5306f15668d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1798982
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695850}