The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f5b9df55357e7d065bc4ecbfda8793840c18880

commit 5f5b9df55357e7d065bc4ecbfda8793840c18880
Author: morlovich <morlovich@chromium.org>
Date: Mon Mar 27 17:54:58 2017

Try to speed up SimpleCache index re-sync on Windows.

FindFirstFile[Ex]/FindNextFile backing FileEnumerator there
already fill in all the information we need, so no need to get
it again via GetFileAttributesEx (backing base::GetFileInfo)

On POSIX, of course, readdir_r doesn't provide file + timestamp info,
so the separate stat is still required.

This helps a lot on my not-quite-micro benchmarks (470ms -> 230ms or so),
but real-life long-tail measurements can be orders of magnitude slower,
so might not have the same root cause. Still, seems worthwhile since
this can be a startup background task reasonably frequently.

(Side note: Windows does seem to have atime, though it's documented as
 basically a*date* on FAT. It may be worth trying to use it to see if it
 helps the hit rate a bit in cases where accurate index info isn't available;
 might be too rare to evaluate, though)

BUG= 705543 

Review-Url: https://codereview.chromium.org/2774923002
Cr-Commit-Position: refs/heads/master@{#459823}

[modify] https://crrev.com/5f5b9df55357e7d065bc4ecbfda8793840c18880/net/disk_cache/simple/simple_index_file.cc
[modify] https://crrev.com/5f5b9df55357e7d065bc4ecbfda8793840c18880/net/disk_cache/simple/simple_index_file.h
[modify] https://crrev.com/5f5b9df55357e7d065bc4ecbfda8793840c18880/net/disk_cache/simple/simple_index_file_posix.cc
[modify] https://crrev.com/5f5b9df55357e7d065bc4ecbfda8793840c18880/net/disk_cache/simple/simple_index_file_win.cc

Stats look good on canary + dev