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To prevent confusion whether delta can be negative.

Change-Id: I6c2f3fc31347c0a0af55fb2b9d91ab364d56370c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1954319
Commit-Queue: Kramer Ge <fangzhoug@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722510}