[iOS][Waffle] Add default search engine icon

It is possible to have no embedded icons for prepopulated search
engines in EEA countries. This happen when the user select
a prepopulated search engine outside of EEA counties, and then switch
to a EEA countries. The selected search engine is still part of
prepopulated search engine, but it doesn't have an embedded icons.

To avoid DCHECK/CHECK issues, the temporary solution is to use
`default_world_favicon` icon. This patch will be merged in M123.

For M124, the code will be update to download icon if it doesn't exist.

Fixed: 326221623
Change-Id: Ic38ea3d94bb918a7fd4e2bc02bb934a8ea658bc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5314600


Reviewed-by: Samar Chehade-Lepleux <samarchehade@google.com>
Reviewed-by: Nicolas Dossou-Gbété <dgn@chromium.org>
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263888}