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Fixes two small issues:
- Padding of visible layer region did not consider page scale.
- New layers may jitter as we won't de-jelly them for 1 frame. We now
  use a reasonable default.

Bug: 995965
Change-Id: Ib2ad58eab9b393b691e586e8649672c1d4323864
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1861096Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705781}