Avoid unnecessary repaints when scrolling composited layers

If everything that moves in response to a scroll offset update is a composited
layer, then we don't need to invalidate in RenderLayerScrollableArea::setScrollOffset. We also don't have to clip the composited bounds (the compositor will do the clipping for us, and if we do the clipping in blink, we have to repaint every time we update the clipped composited bounds).

TEST=
virtual/softwarecompositing/geometry/limit-layer-bounds-clipping-ancestor.html
compositing/geometry/limit-layer-bounds-clipping-ancestor.html
compositing/overflow/do-not-repaint-if-scrolling-composited-layers.html

BUG=307114

Review URL: https://codereview.chromium.org/27239004

git-svn-id: svn://svn.chromium.org/blink/trunk@160774 bbb929c8-8fbe-4397-9dbb-9b2b20218538