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This CL is just the boilerplate of creating new classes and wiring them
up to WebAppProvider.

Eventually, the functionality from web_app_extension_shortcut should be
migrated into this class. A WIP CL for beginning this is available here:
https://crrev.com/c/1846422.

The motivation for beginning this now is file handling on Linux, as
registering and unregistering file handlers in the OS is tightly
coupled to shortcut management.

Bug: 860581
Change-Id: I3e28018d6fd6656fccd3c22ed62b9576db43f51c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847708
Commit-Queue: Jay Harris <harrisjay@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704484}