Propagate writing-mode from the first region to the flow thread.

Since the flow thread is a direct child of RenderView, it doesn't inherit
proper writing-mode automatically. It should be mentioned that if the thread's
contents' writing-mode differs from that of the first region, things are
typically going to look useless (although perhaps that's how it should be),
but as long as writing-mode is only specified on a common parent of the
thread's contents and the regions, things look fine, and also, we're now
following what the spec has to say on the matter:

http://www.w3.org/TR/2013/WD-css3-regions-20130528/#the-flow-into-property

    "The first region defines the principal writing mode for the entire flow.
     The writing mode on subsequent regions is ignored."

BUG=257965

Review URL: https://chromiumcodereview.appspot.com/18374008

git-svn-id: svn://svn.chromium.org/blink/trunk@155779 bbb929c8-8fbe-4397-9dbb-9b2b20218538