Images in feed on ebay.com jiggle when one is hovered

Images in feed on ebay.com jiggle when one is hovered
https://bugs.webkit.org/show_bug.cgi?id=155608
<rdar://problem/25160681>

The content offset in compositing layer = subpixel gap between the graphics layer and the layer bounds + layer bounds top left.

Reviewed by Simon Fraser.

Source/WebCore:

Test: compositing/hidpi-viewport-clipping-on-composited-content.html

* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::updateGeometry):
(WebCore::RenderLayerBacking::contentOffsetInCompostingLayer):
* rendering/RenderLayerBacking.h:

LayoutTests:

* compositing/hidpi-viewport-clipping-on-composited-content-expected.html: Added.
* compositing/hidpi-viewport-clipping-on-composited-content.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@198374 268f45cc-cd09-0410-ab3c-d52691b4dbfc