Move LineWidth class and related code into LineWidth.{h,cpp}.

RenderBlockLineLayout.cpp is getting bigger and bigger, it's hard to find and modify things in it. The first goal would be to separate the helper classes to their own files to reduce its complexity. The helper classes will be used by the ongoing refactoring of the RenderBlockFlow later.

Merged from WebKit:
 - http://trac.webkit.org/changeset/155565
 - http://trac.webkit.org/changeset/155569

Review URL: https://chromiumcodereview.appspot.com/23902028

git-svn-id: svn://svn.chromium.org/blink/trunk@157806 bbb929c8-8fbe-4397-9dbb-9b2b20218538