CodeHealth: Add check_empty=true to ObserverLists in //c/b/web_applications

Bug: 1447952
Change-Id: I930e29f1865b61312ac3ad1a96f4d7e798d04268
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4560765


Auto-Submit: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148389}