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This CL adds support so File Handling origin trial can be used for
System Web Apps.

Under the hood, SystemWebAppManager force enables file handlers after
Apps are installed (in on_apps_synchronized event). FileHandlerManager
treats force enabled file handling origin trial to never expire
(technically, it expires at the max time that can be persisted in
preferences).

Fixed: 1045297
Change-Id: Iaa55601c3ecbe37c543beb1ee6f495b62fac6617
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2093959
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749541}