GitLab

In this CL:
- Save FileHandlers to WebAppDatabase
- Implement GetAllFileHandlers in WebAppFileHandlerManager

Bug: 938103
Change-Id: I64a2115dc800b8216f7899d9f83ae19586b45db7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1958297
Commit-Queue: Robert Woods <robertwoods@google.com>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727279}