When the set of backing-sharing layers changes, we need to issue a repaint

When the set of backing-sharing layers changes, we need to issue a repaint
https://bugs.webkit.org/show_bug.cgi?id=197825

Reviewed by Zalan Bujtas.

Source/WebCore:

If the set of layers painting into a shared backing store changes, we need
to repaint that backing store. This happens when scrolling as shared layers
enter the visible area.

Test: compositing/shared-backing/overflow-scroll/repaint-shared-on-scroll.html

* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::setBackingSharingLayers):

LayoutTests:

* compositing/shared-backing/overflow-scroll/repaint-shared-on-scroll-expected.html: Added.
* compositing/shared-backing/overflow-scroll/repaint-shared-on-scroll.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@245220 268f45cc-cd09-0410-ab3c-d52691b4dbfc