Remove includes from LineWidth.h

We don't need to include RenderBlock.h and RenderRubyRun.h in LineWidth.h,
so I added forward declarations, and I moved the includes to LineWidth.cpp.

Review URL: https://codereview.chromium.org/27903007

git-svn-id: svn://svn.chromium.org/blink/trunk@160165 bbb929c8-8fbe-4397-9dbb-9b2b20218538