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This lets web apps that can handle files show up in the
open-with menu on Linux.

Note: This is still experimental and hidden behind the
NativeFileSystemAPI and FileHandlingAPI flags.

In addition, some work needs to be done to register custom mimetypes
with the operating system as in:
https://specifications.freedesktop.org/shared-mime-info-spec/latest/ar01s02.html

Bug: 829689
Change-Id: I92bcc07cb5dffaa715c409bc6cf07c8410db34ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833027
Commit-Queue: Jay Harris <harrisjay@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710593}