Separate BreakingContext from RenderBlockLineLayout.cpp

This patch is part of the bigger RenderBlockLineLayout refactoring (tracking bug: https://code.google.com/p/chromium/issues/detail?id=316347).
In this patch I introduced the line subdirectory in core/rendering, which will contain all of the linelayout related files. In this patch I
added BreakingContextInlineHeaders.h and separated the LineInfo class into LineInfo.h. BreakingContextInlineHeaders contains more helper classes
which will require their own files, but I'll do that later.

BUG=312826

Review URL: https://codereview.chromium.org/66043002

git-svn-id: svn://svn.chromium.org/blink/trunk@161839 bbb929c8-8fbe-4397-9dbb-9b2b20218538