Clipping along compositing borders in svg-edit

Clipping along compositing borders in svg-edit
https://bugs.webkit.org/show_bug.cgi?id=151791

Reviewed by Zalan Bujtas.

Source/WebCore:

RenderSVGRoot::layout() failed to clear overflow before recomputing
visual overflow, which could cause it to get stuck with stale overflow.
This would cause underpainting if its size went from small to large.

Test: svg/overflow/visual-overflow-change.html

* rendering/svg/RenderSVGRoot.cpp:
(WebCore::RenderSVGRoot::layout):

LayoutTests:

Ref test that triggers recomputation of overflow.

* svg/overflow/visual-overflow-change-expected.html: Added.
* svg/overflow/visual-overflow-change.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@193613 268f45cc-cd09-0410-ab3c-d52691b4dbfc