The main intention of this change is to have a consistent look and
behavior across all scrollbars, including elements with
`overflow: scroll` and `overflow: auto`, iframes, and a page.
Before:
- Page's scrollbar is painted by Browser (Qt/AppKit) using the
corresponding UI framework style,
- Both WebContent and Browser know the scroll position offset.
- WebContent uses did_request_scroll_to() IPC call to send updates.
- Browser uses set_viewport_rect() to send updates.
After:
- Page's scrollbar is painted on WebContent side using the same style as
currently used for elements with `overflow: scroll` and
`overflow: auto`. A nice side effects: scrollbars are now painted for
iframes, and page's scrollbar respects scrollbar-width CSS property.
- Only WebContent knows scroll position offset.
- did_request_scroll_to() is no longer used.
- set_viewport_rect() is changed to set_viewport_size().
By replacing the `page_did_request_scroll_to()` calls with a request
to perform scrolling in the corresponding navigable, we ensure that
the scrolling of iframes will scroll within them instead of triggering
scroll of top level document.
