Layer squashing causes incorrect rendering while scrolling in Gmail

RenderLayer::boundingBoxForCompositing knew how to exclude RenderLayers that
had their own composited layer mapping, but it didn't know how to exclude
RenderLayers that paint into grouped backings.

R=vollick@chromium.org
BUG=370667

Review URL: https://codereview.chromium.org/271863003

git-svn-id: svn://svn.chromium.org/blink/trunk@173653 bbb929c8-8fbe-4397-9dbb-9b2b20218538