[MemCache] Fix bug while iterating LRU list in eviction

It was possible to reanalyze a previously doomed entry.

Bug: 827492
Change-Id: I5d34d2ae87c96e0d2099e926e6eb2c1b30b01d63
Reviewed-on: https://chromium-review.googlesource.com/987919
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547236}
net::CacheType MemBackendImpl::GetCacheType() const {
  return net::MEMORY_CACHE;
}

net::CacheType MemBackendImpl::GetCacheType() const {
  return net::MEMORY_CACHE;
}
