Make InlineBox::renderer() and related subclass methods return reference.

It always created like InlineBox(*this) so it always has a renderer.
Also removes NULL checks and asserts which are not longer required.

BUG=

Review URL: https://codereview.chromium.org/184023003

git-svn-id: svn://svn.chromium.org/blink/trunk@168237 bbb929c8-8fbe-4397-9dbb-9b2b20218538