Move BreakingContext out of LineBreaker

This patch just moves out BreakingContext from the scope of LineBreaker. The goal is to separate BreakingContext
into its own file (https://chromiumcodereview.appspot.com/25054004#msg2). I want to do this step by step. This change
moves out BC and leaves it in place. I'll make a separate patch to add the new file for it.

Review URL: https://codereview.chromium.org/39003002

git-svn-id: svn://svn.chromium.org/blink/trunk@160766 bbb929c8-8fbe-4397-9dbb-9b2b20218538