RootInlineBox should deal with RenderBlockFlow, not RenderBlock

Part of the ongoing RenderBlockFlow refactor. InlineBoxes should be held
onto by BlockFlows, not the more generic Blocks. As such, RootInlineBox
should coonsider its block pointers RenderBlockFlows.

BUG=302024

Review URL: https://codereview.chromium.org/53143003

git-svn-id: svn://svn.chromium.org/blink/trunk@161962 bbb929c8-8fbe-4397-9dbb-9b2b20218538