Untangle NeedsFullPaintInvalidationInBacking from updateLayerPositionsAfterLayout

There's no reason to pass NeedsFullPaintInvalidationInBacking into
updateLayerPositionsAfterLayout. We can just repaint all the composited layers
using RenderLayerCompositor::fullyInvalidatePaint(), which does a separate
walk of the RenderLayer tree.

I've also changed RenderLayerCompositor::fullyInvalidatePaint() to walk in tree
order instead of in z-order. There's no reason to walk in z-order and walking in
tree order is faster.

R=esprehn@chromium.org
BUG=382548

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179538

Review URL: https://codereview.chromium.org/438173002

git-svn-id: svn://svn.chromium.org/blink/trunk@179568 bbb929c8-8fbe-4397-9dbb-9b2b20218538