[dPWA] Reduce the code size of WebAppUninstallJob

This CL is a code clean up of WebAppUninstallJob that reduces the amount
of boilerplate involved.

This also updates the call to MaybeScheduleProfileForDeletion() to wait
for it to complete before completing the WebAppUninstallJob.

Bug: 1427340
Change-Id: Ia5f8ddb9f0eca8a6cf1c206a1162d2397db57ee7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546947


Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148382}