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The average_lag_tracker assumes that all events have nondecreasing
timestamps. However, the assumption breaks when the main thread takes a
long time to handle a scroll update and the compositor thread starts a
new scroll in the meantime.

This CL removes the invalid DCHECK and instead ignores the out-of-order
event.

Bug: 1015492
Change-Id: I3dcee2be9c0ba9b6e295d391b4c114f7ef05510b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869378Reviewed-by: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Ella Ge <eirage@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#712910}