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This CL adds another field predicted_scroll_update_delta to keeps the
scroll delta after prediction. And also updates the AverageLag to
take the predicted_scroll_update_delta into account.

Bug: 946212
Change-Id: I1ba2bc5bd644c630d53d7f02d6ab4c4c4259f09b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1549346
Commit-Queue: Ella Ge <eirage@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648709}