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In https://crrev.com/624999, fixed and sticky positioned elements were
promoted if they stick to a scroller. This patch takes that further and
promotes fixed and sticky elements if they stick to a scroller, even if
the scroller is overflow: hidden. This will let us remove the
kHasNonLayerViewportConstrainedObjects main thread scrolling reason in a
followup, which is difficult to support with CompositeAfterPaint.

It is possible to have scroll animations with overflow hidden,
scroll-behavior: smooth, and the root scroller.

This is expected to regress the ie_chalkboard benchmark which now gets
layerized which requires a raster larger than the visible rect. Before
this patch, only the visible rect would be re-rasterized because there
were no layers. This situation should be rare and this patch should be
a small progression overall because it reduces main thread scrolling
reasons.

Bug: 649096
Change-Id: I6b1b15584166ace3dcd9b40d8271b13fb12b6366
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2100113
Commit-Queue: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Auto-Submit: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842312}