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This is a reland of a358e673

Original change's description:
> Adds mojo_base.mojom.ReadOnlyFile
>
> Adds a mojo ReadOnlyFile that won't serialize if it can be written.
>
> The expectation is that this handle will then be sent to another process
> which lacks the rights to add any write permissions to the handle. This
> may not be true on all platforms but is useful where this holds.
>
> Code to check for readonly-ness is implemented within mojo rather than
> base::File as it is (a) difficult to say in general what it means to be
> readonly, and (b) not possible on every platform.
>
> Bug: 1130762
> Change-Id: I6f00934005da7e433af4fee94454dfcd28dfd3c5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417549
> Commit-Queue: Alex Gough <ajgo@chromium.org>
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Ken Rockot <rockot@google.com>
> Reviewed-by: Matthew Denton <mpdenton@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#813987}

Bug: 1130762
Change-Id: I4ae14a37b57867e6b02ad2e3f69d248032e0c3c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450893Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814537}