Move RenderBlockFlow::constructTextRun out.

It doesn't need anything particular to block flows, and is used by other
kinds of renderers.

Review URL: https://codereview.chromium.org/397723006

git-svn-id: svn://svn.chromium.org/blink/trunk@178467 bbb929c8-8fbe-4397-9dbb-9b2b20218538