Have Document::formController() return a reference

Have Document::formController() return a reference as it can never return null.
It makes the code look safer.

R=adamk

Review URL: https://codereview.chromium.org/181653002

git-svn-id: svn://svn.chromium.org/blink/trunk@167928 bbb929c8-8fbe-4397-9dbb-9b2b20218538