Add placeholder implementation of DOM `VisualViewport` according to the
spec https://drafts.csswg.org/cssom-view/#visualviewport. This is the
first step of implementing the interface and syncing it with the
`PinchZoom` struct that we had in the `Embedder`.
Since this interface's is not accurate and couldn't fulfill it's
purpose, it would be gated behind a preference
`dom_visual_viewport_enabled` and would be open after the interface had
been integrated with `PinchZoom`, viewport resizing, and viewport
scroll.
Testing: Existing WPT Test.
Part of: #41341
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Signed-off-by: Jo Steven Novaryo <steven.novaryo@gmail.com>
