[Image Redux] Use the negotiated size of a background image for the visually non-empty pixel count

https://bugs.webkit.org/show_bug.cgi?id=326359

Reviewed by Darin Adler.

Replace use of Style::Image::imageSize() with the appropriate negotiated size for background images
when calling incrementVisuallyNonEmptyPixelCountIfNeeded() in RenderBox::imageChanged().

* Source/WebCore/rendering/RenderBox.cpp:

Canonical link: https://commits.webkit.org/322762@main