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This CL improves the jank metrics by excluding the delays caused by
frames without intended updates and prevents certain outlier cases
when a high jank percent is reported on a page with very low percent
of dropped frames.

Bug: 1133058
Change-Id: If32576e8b528e22b184a0b549161751fcfd976bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417211Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Mingjing Zhang <mjzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812988}