Don't send scroll events in case of new picture from webkit.

When user goes back to a page, the new picture may potentially cause
a scroll event which overrides the history scroll position, since
the scroll position may be bigger than the initial new picture.

Setting the scroll event to be not-user-scrolled will preserve the
history scroll position.

issue: 3433562
Change-Id: Ida176a4bc3a22de0385aed71e11f8b8e0db06f24