[CSS Regions] Layout Test for selecting text in 2 regions

Patch already landed in WebKit. See http://trac.webkit.org/changeset/156857 and
http://trac.webkit.org/changeset/156914

Add new reftests for selecting text in 2 regions. It compares the
behavior (highlighted text and selected content) in regions and absolute
positions.

Different layout tests for other writing modes are provided too.

Review URL: https://codereview.chromium.org/25890007

git-svn-id: svn://svn.chromium.org/blink/trunk@158985 bbb929c8-8fbe-4397-9dbb-9b2b20218538