Periodically repaint during resize while using the DynamicSizeWithMinimumViewSize...

Periodically repaint during resize while using the DynamicSizeWithMinimumViewSize layout strategy
https://bugs.webkit.org/show_bug.cgi?id=144816

Reviewed by Simon Fraser.

* UIProcess/mac/WKViewLayoutStrategy.mm:
(-[WKViewDynamicSizeWithMinimumViewSizeLayoutStrategy initWithPage:view:mode:]):
(-[WKViewDynamicSizeWithMinimumViewSizeLayoutStrategy _updateTransientScale:]):
(-[WKViewDynamicSizeWithMinimumViewSizeLayoutStrategy updateLayout]):
Factor out _updateTransientScale from updateLayout.
Keep track of the last viewScaleFactor that we know is being displayed
(_lastCommittedViewScale) and use that for computing the transient scale,
so that we can recompute the transient scale while the UI process's notion
of the actual view scale might have moved ahead of what the Web process has
painted.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@184022 268f45cc-cd09-0410-ab3c-d52691b4dbfc