Avoid doing an extra layout in some cases while doing scale-to-fit

Avoid doing an extra layout in some cases while doing scale-to-fit
https://bugs.webkit.org/show_bug.cgi?id=145321
<rdar://problem/21051165>

Reviewed by Beth Dakin.

* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.h:
* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.mm:
(WebKit::TiledCoreAnimationDrawingArea::scaleViewToFitDocumentIfNeeded):
If we make it to scaleViewToFitDocumentIfNeeded with valid layout,
the view width is still less than the last-known minimum document size,
and the document size hasn't changed, update the view scale without
doing an extra layout, keeping the fixed layout size at the previously
computed minimum document size.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@184797 268f45cc-cd09-0410-ab3c-d52691b4dbfc