Blockfile cache: fix long-standing sparse + evict reentrancy problem

Thanks to nedwilliamson@ (on gmail) for an alternative perspective
plus a reduction to make fixing this much easier.

Bug: 826626, 518908, 537063, 802886
Change-Id: Ibfa01416f9a8e7f7b361e4f93b4b6b134728b85f
Reviewed-on: https://chromium-review.googlesource.com/985052
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547103}
void BackendImpl::SyncEndEnumeration(
    std::unique_ptr<Rankings::Iterator> iterator) {
  iterator->Reset();
}

void BackendImpl::SyncEndEnumeration(
    std::unique_ptr<Rankings::Iterator> iterator) {
  iterator->Reset();
}
