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Adds an ErrorPolicy setting to base::FileEnumerator. By default
(ErrorPolicy::IGNORE_ERRORS) all FileEnumerators behave as they
always have. If set to ErrorPolicy::STOP_ENUMERATION however, a
FileEnumerator will cease enumeration as soon as it encounters
any error, and the corresponding error code can be retrieved from
the FileEnumerator.

The motivation for this change is that some existing logic in
third_party/leveldb/env_chromium.cc duplicates most of
FileEnumerator's behavior just to work around the lack of error
reporting. This change will allow the code to be de-duplicated
without disturbing error reporting histograms.

Bug: 1052045
Change-Id: I0639029403fe42d1baef12075898e3cc9ec7f6a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2082559
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746988}