Clear scrollbar damages in time

When scrollbar is invalidated during layout, the damaged area is cached
so that the next invalidateTreeIfNeeded will pick up them and do real
invalidation.

However, when we do full view repaint we skip invalidation of scrollbar
damages, but leave the damage status not cleared.

Override RenderObject::clearPaintInvalidationState() and
RenderObject::paintInvalidationStateIsDirty() to handle clearing of
scrollbar damage status.

BUG=394833
TEST=fast/repaint/vertical-overflow-parent.html
TEST=fast/repaint/scrollbar-damage-and-full-viewport-repaint.html
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/403593005

git-svn-id: svn://svn.chromium.org/blink/trunk@179433 bbb929c8-8fbe-4397-9dbb-9b2b20218538