Consolidate shouldMoveToFlowThread() into RenderTreeBuilder

This patch removes the callback from Element and SVGElement and puts the logic
into RenderTreeBuilder. This means less of the code base needs to know about
regions and also removes another "C++ backdoor" so that future elements don't
use this hook and could possibly be implemented as web components.

Review URL: https://codereview.chromium.org/127753004

git-svn-id: svn://svn.chromium.org/blink/trunk@164804 bbb929c8-8fbe-4397-9dbb-9b2b20218538