Ensure positioned descendants are invalidated if needed

When a render object changes location, we should force paint
invalidation checking for all descendants on the same backing,
including positioned ones. However, previously we only force
invalidation checking for non-positioned descendants.

Now let RenderBlock override
RenderObject::invalidatePaintOfSubtreesIfNeeded() so that it
can use the childPaintInvalidationState, which has been forced
checking of invaldation, for the positioned descendants.

BUG=422812
TEST=fast/repaint/positioned-great-grandparent-change-location.html

Review URL: https://codereview.chromium.org/662483002

git-svn-id: svn://svn.chromium.org/blink/trunk@183910 bbb929c8-8fbe-4397-9dbb-9b2b20218538