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This CL fixes an issue introduced in https://crrev.com/c/1999288.
Scrollers can set |user_scrollable_horizontal| (or vertical) to false
to indicate that the user shouldn't be able to scroll them  This
corresponds to |overflow: hidden| in CSS (which can still be
programmatically scrolled). In several places, compositor code checks
these bits and, if false, clears the requested scrolling delta in the
disabled axis.

The bug in the CL above was that, now that the scrolling paths were
consolidated,  we moved this "user_scrollable" adjustment from the
update animation curve to happen as part of the common path in
ScrollUpdate so that it would happen for all kinds of scroll modalities
in one place.

However, this adjustment is too simplistic for viewport scrolling.
Scrolling the outer viewport must use the cc::Viewport class which will
distribute the scroll between both inner and outer scroll nodes. The
outer viewport may have user_scrollable=false, corresponding to the
documentElement having |overflow: hidden| but we should still be able
to scroll the inner viewport if the user zooms in. The above adjustment
prevents that since it clears the delta. (I suspect this issue existed
prior to this in the "update existing animation" case but is hard to hit
since pinch-zoom and animated wheel scrolling aren't a common
combination).

The solution here is to move these adjustments to the point of node
scrolling. This means that we'll adjust the delta right before applying
a scroll to an individual ScrollNode. This ensures the entire delta is
sent to the viewport which has special cases for dealing with the double
nodes.

Bug: 1055790
Change-Id: I2e9849b15c512003352299fc82e58817a98ef778
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072407
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745229}