WebApp: Implement InstallFinalizer::UninstallExternalWebApp by app_id.

Introduce new virtual UninstallExternalWebApp function.
InstallFinalizer::UninstallExternalWebAppByUrl is implemented in terms of
virtual UninstallExternalWebApp function.

Bug: 876576
Change-Id: Ica9a47630cfdce0421ceae2816efadc8d5b95e95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1999969
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731143}
9 files changed