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This bug occurs because of Clank's "resize main frame after layout"
behavior. In a nutshell: we want users to be able to zoom out to that
the entire width of the content fits on screen. Because we can't zoom
out further than the layout viewport (i.e. the main FrameView), after
a layout occurs we resize the main FrameView to match the content width.
This leaves us in an interesting situation - the viewport scroller is
actually larger than the initial containing block (i.e. the LayoutView
size). This situation cannot occur on desktop and generally doesn't
occur on well-behaved mobile sites (sites that don't allow zoom out).

The bug was occuring because the intersection observer used the
LayoutView/ICB size for the root element. This is incorrect in the
situation described above. We should really use the ViewRect which is
the scroller bounds.

Bug: 836408
Change-Id: I96029cdb11a6a16353d5b8b628b0182c13926a8d
Reviewed-on: https://chromium-review.googlesource.com/1031118Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554661}