Per email conversation with lod@ and sdefresne@

icudtl.dat needs to be filtered out by passing the following to the ios_appex_bundle target:

bundle_deps_filter = [ "//third_party/icu:icudata" ]

It looks like neither content_widget_extension nor search_widget_extension

ICU may also need to be configured similarly to the today_extension in line 277-283 of today_view_controller.mm.