Refactor drawLineForBoxSide into several smaller methods.

The case statement bodies are independant units, moved each one out into
its own method to make things easier to understand.

BUG=305038

Review URL: https://codereview.chromium.org/26258002

git-svn-id: svn://svn.chromium.org/blink/trunk@159117 bbb929c8-8fbe-4397-9dbb-9b2b20218538