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This functionality will be used by the ContextProvider service to
bind an arbitrary, client-supplied directory handle to the "/data"
directory of the child Context processes.

* Adds a helper function for extracting file handles from File objects.
* Adds unit tests.
* Migrates some uses of ScopedZxHandle to zx::handle.

Bug: 850743, 852541
Change-Id: Iada1775485745124822e32b9ac81aefaff6b9059
Reviewed-on: https://chromium-review.googlesource.com/1091803
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567880}