Push selection gaps logic down to RenderBlockFlow

Part of my ultimate goal of de-coupling invalidation and painting of selection
gaps from one function so I can move the painting into BlockFlowPainter.

We don't actually paint selection gaps for non-blockFlow RenderBlocks. Make the
code actually represent that!

Review URL: https://codereview.chromium.org/720313002

git-svn-id: svn://svn.chromium.org/blink/trunk@185396 bbb929c8-8fbe-4397-9dbb-9b2b20218538