Mark values if their corresponding key was already marked. Also
take into account the size for backing store.

Bug: chromium:844008
Change-Id: I58570f935c69fb39b686fd22dc93caba309f83b2
Reviewed-on: https://chromium-review.googlesource.com/1097400Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53690}