[MemCache] Fix bug while iterating LRU list in range doom

This is exact same thing as https://chromium-review.googlesource.com/c/chromium/src/+/987919
but on explicit mass-erase rather than eviction.

Thanks to nedwilliamson@ (on gmail) for the report and testcase.

Bug: 831963
Change-Id: I96a46700c1f058f7feebe038bcf983dc40eb7102
Reviewed-on: https://chromium-review.googlesource.com/1014023
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551205}