When sweeping is in progress, either finalize or do nothing

BUG=none
LOG=n
R=rmcilroy@chromium.org, hpayer@chromium.org

Review URL: https://codereview.chromium.org/1070653003

Cr-Commit-Position: refs/branch-heads/4.3@{#4}
Cr-Branched-From: f5c0a23a-refs/heads/4.3.61@{#1}
Cr-Branched-From: 0a7d4f49-refs/heads/master@{#27508}