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This rewrites ChromiumEnv to use the storage::FilesystemProxy
API for all filesystem access. By default it will use unrestricted
instances with direct filesystem access, effectively equivalent to
the status quo. No interesting behavioral or performance changes
are expected here.

This will allow a follow-up CL to implement sandboxing support for
LevelDB by constructing a ChromiumEnv instance over a restricted
FilesystemProxy instance.

Bug: 1052045
Change-Id: Ic23726b3f5abd3f1a322b49827e80dbf424c3de7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080673
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747584}