No longer store scroll offsets for elements in the DOM. Instead
consistently get and set these in layout's `ScrollTree`. This more
consistently requires layout to run when querying scroll offsets, which
ensures that they are up-to-date and properly bounded by scrollable
overflow area.
Testing: This causes several WPT tests to start passing, and one to
start
failing. In the case of
`/shadow-dom/scroll-to-the-fragment-in-shadow-tree.html`, I believe the
issue
is that we don't properly handle scrolling and shadow DOM elements.
Before, the
faulty scrolling was hiding this issue.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
