Display anonymous regions in DRT

This patch allows displaying the anonymous region inside a block with flow-from style set.

Before the patch:
  RenderRegion {DIV} at (200,200) size 52x52 [border: (1px solid #000000)]
After the patch:
  RenderBlock (positioned) {DIV} at (200,200) size 52x52 [border: (1px solid #000000)]
    RenderNamedFlowFragment at (1,1) size 50x50

Before the patch:
  Flow Threads
    Thread with flow-name 'article'
      Regions for flow 'article'
      RenderRegion {DIV} #region_1
      RenderRegion {DIV} #region_2

After the patch:
  Named flows
    Named flow 'article'
      Regions for named flow 'article'
        RenderNamedFlowFragment (anonymous child of {DIV} #region_1)
        RenderNamedFlowFragment (anonymous child of {DIV} #region_2)

BUG=321217

Review URL: https://codereview.chromium.org/79433004

git-svn-id: svn://svn.chromium.org/blink/trunk@162606 bbb929c8-8fbe-4397-9dbb-9b2b20218538