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This CL takes advantage of the fact that NativeIOQuotaClient is now
mojofied and no longer ref-counted, and simpifies NativeIO's ownership
graph.

Bug: 1016065
Change-Id: I6eb6fa259111101e95459b5c0655261b0d7e43d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644638Reviewed-by: Jarryd Goodman <jarrydg@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846323}