desktop-pwas: Move uninstallation to helper class

Removes usage of ExtensionSystem and ExtensionService from
PendingBookmarkAppManager by moving uninstallation code to a separate
class.

This brings PendingBookmarkAppManager closer being just a queue of tasks.

Bug: 916381
Change-Id: I575fede7203af0028ed4dcc95044edd7aab53dab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1514272
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639722}
5 files changed