Minimize RenderObject* casting to RenderText*

Minimize RenderObject* casting to RenderText* by providing renderer() methods
on Text and InlineTextBox that return a RenderText. This hides the more generic
renderer() methods in their parent that return a RenderObject.

This is inspired by WebKit r157373 by antti@apple.com:
https://trac.webkit.org/r157373

R=esprehn@chromium.org, pdr@chromium.org

Review URL: https://codereview.chromium.org/414863002

git-svn-id: svn://svn.chromium.org/blink/trunk@178991 bbb929c8-8fbe-4397-9dbb-9b2b20218538