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In http://crbug.com/881384#c2 I saw some more evidence that the page cache is
not fully flushed when benchmarking. On Linux/Android calling sync(2) syscall
before evicting individual files should help because the later pagecache
dropping operations become a lot cheaper for the kernel. I could not find good
documentation about sync(2) guarantees on MacOS.

The page cache flushing for individual files is still asynchronous, with no
cheap way to check that it happened. The plan is to sleep for one second after
such operations. Since we would prefer to sleep only once per bulk of
operations, the sleeping will be done on higher levels.

The Windows story is not covered - the operation requires administrator
privileges.

Bug: 811244
Change-Id: Ib71f4f934a0eb4b438bfedbbb50462e07ff64308
Reviewed-on: https://chromium-review.googlesource.com/1211602
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590266}