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This CL fixes a bug when scrolling from outside the root scroller
hierarchy. The root scroller is the "main" scroller on a page. On
Android, this can be designated on an element other than the document.
However, it's still possible to scroll a chain of elements outside of
the root scroller subtree. For example, from a sibling of the root
scroller or from a |position: fixed| element. In these cases, when the
scroll chains to the root, we still want to scroll the visual viewport
so that we support panning around if the user is zoomed in. We don't do
this on the main thread prior to this CL.

Typically, the VisualViewport is scrolled by using the RootFrameViewport
class. However, in this case we can't use it since that would cause
scrolling in the root scroller (and thus change behavior based on
whether the page is viewed using a UA that supports root scroller). This
CL adds a special case to Node::NativeApplyScroll. When we construct a
chain that doesn't end at the root scroller, we'll always add the root
LayoutView to the scroll chain. When scrolling it, we'll attempt to
scroll the visual viewport if the document itself can no longer scroll.

Note: we have an equivalent special case in
LayerTreeHostImpl::ApplyScroll where we can pass a bool to
viewport()->ScrollBy() to specify that we should scroll only the visual
viewport. This behavior is somewhat more natural in CC since the scroll
chain walks the ScrollTree which contains the visual viewport. Blink's
scrolling code assumes all scrollable nodes are DOM nodes which isn't
true of the visual viewport; hence the need to make this a special case
of the root document node.

This was implemented in CC in https://crrev.com/c/1752866. At the time,
it was assumed this case would be exceedingly rare on the main thread so
it was left unimplemented. However, fixing bug 1011866 will result in
this case becoming more common.

Bug: 977954, 1011866
Change-Id: I7d2d9cb70b66b54a8c972af4bdc6d69978fe690b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859940Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707222}