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When generating input events for fling updates, use the current
animation time instead of the current clock ticks. This ensures latency
measurements for fling animation frames are correctly aligned with
BeginFrame timestamps.

We've compared the computation of the deltas which are producer for
the scrolls for various phones to ensure this is an improvement in
animation smoothness.

The thing to note when looking at these screenshots are the smoothness of
the curve (ignoring the flat parts).

For more details about how these are generated see internal bug: b/149825814

Nokia: https://drive.google.com/file/d/1Y2aPWatHg6ON98HosY9KYrgLM2SeGT_j/view?usp=sharing
Pixel2: https://drive.google.com/file/d/1Va7ojVlQdk8NLdtCjdxntU5zjMUFlgbP/view?usp=sharing
Pixel4: https://drive.google.com/file/d/1iazS1N1odhOqpGWTEjh_VRFi0VubZYwi/view?usp=sharing

Change-Id: I7383539605f9588640c86b7342b4d93f0d65ccb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2052174
Commit-Queue: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746722}