macOS V2 Sandbox: Harden sandbox by specifying vnode-type for writes.

This hardens the macOS sandbox by specifying an explicit vnode type for
all writeable files, reducing the risk that an attacker can symlink a
file Chrome has write access to, and modify the filesystem at will.

Bug: 793402
Change-Id: I53bbcc84345e432756eff0d2dc18a505719fd521
Reviewed-on: https://chromium-review.googlesource.com/829877


Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Greg Kerr <kerrnel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524413}