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When performing snapping on a root scroller that is not the main
frame's layout viewport we now perform snapping using the
RootFrameViewport instead of only using the layout viewport (the
PaintLayerScrollableArea).

Also added a test to check this behaviour.

Bug: 1015451
Change-Id: I2276b904adef1bb970bd290b49d44ac5e7153982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865048
Commit-Queue: Kaan Alsan <alsan@google.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709190}