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Programmatic smooth scrolls should also be instant if the smooth scroll
flag is explicitly disabled.

Previously landed in https://crrev.com/c/1536759 and in
https://crrev.com/c/2009842

To reland, this CL force enables the scroll animator in Blink web
tests. Without this, the usual flow for determining if the animator is
enabled is followed, which leads to inconsistencies. On Mac, the
animator is enabled in Chrome but not in content shell. This leads to
inconsistencies with other platforms (Mac tests ran without smooth
scroll while Linux/Windows ran with) and ensuring we test what we ship.

This change required some minor fixes to tests that assumed instant
scroll on Mac (and uncovered one real bug).

Second reland fixes a few more tests on Mac. See diff from original
patchset for changes.

TBR=flackr@chromium.org,nasko@chromium.org

Bug: 944583
Change-Id: Ib91c417f1fcc2b27601f2c7039b02da72b9d5420
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2022356Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735480}