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After https://crrev.com/c/2588910 introduced a NativeIOManager in
content/browser, this CL renames Blink's NativeIOManager to
NativeIOFileManager to avoid future confusion.

No functional changes are introduced.

Bug: 1137788
Change-Id: I190ec89ac33864ce347374a33a0d6e57cb21f434
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593374Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Richard Stotz <rstz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837996}