Add unit tests for web app scope extension updates.

Introduces `web_app_scope_extensions_unittest.cc` to verify that the
`WebAppEffectiveScopeChangedDelegate` observer is triggered when web app
scope extensions are added or removed via reinstallation or manifest
silent updates. Also updates the dynamic scope toolbar plan to be
completed.

Change-Id: Ic54c6ede0bb2412bb72c64a84fa439995c9e8af7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6941655
Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: May Siem <msiem@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519731}