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Instead of going through mojom::blink::FileUtilitiesHost and passing a path
from the renderer back to the browser, add a new method to mojom::blink::Blob
to do the same thing. Since the blob already knows what file a blob is
referencing, this both makes things slightly more secure (or at least gets
us closer to a place where we can get rid of things like
ChildProcessSecurityPolicyImpl::CanReadFile) and makes this
work in a couple more cases after a File is for example postMessaged to a
worker or frame in a different process as well.

Additionally this gets us closer to unifying what the renderer and browser
processes consider to be "the" snapshot state of any particular blob. In
a follow-up CL the browser side code will change to return its snapshot
state (if any), rather than always returning the current state on disk.

This is part of a series of changes to improve consistency surrounding how
snapshot state of files/blobs is implemented.

Bug: 844874
Change-Id: Ia05cfd21a3d78955ff70933995e93bbe59e3bb3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1970301
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726995}