Scroll position jumps when scrolling scaling pages down in split view

Scroll position jumps when scrolling scaling pages down in split view
https://bugs.webkit.org/show_bug.cgi?id=147072
rdar://problem/21769577

Reviewed by Sam Weinig.

When apple.com/music is in Split View at a reduced scale, scrolling the page
can cause unwanted scroll jumps. This happened because we'd enter
TiledCoreAnimationDrawingArea::scaleViewToFitDocumentIfNeeded() with a pending
layout, so run the autosizing logic. When scrolled near the bottom, the unconstrained
layout resulted in a shorter document, which truncated the scroll position. The
scaled layout then restored the longer document, but it also restored that
truncated scroll position.

Forcing a layout is sufficient to fix the bug for this page, and doing a single
layout (which will happen anyway) is preferable to running the autosize logic
if layout happens to be dirty.

I was not able to make a test to reproduce the problem. Entering
scaleViewToFitDocumentIfNeeded() with pending layout was triggerable
using a transform transition, but I was not able to reproduce incorrect
scroll position restoration.

* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.mm:
(WebKit::TiledCoreAnimationDrawingArea::scaleViewToFitDocumentIfNeeded):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@187001 268f45cc-cd09-0410-ab3c-d52691b4dbfc