Avoid useless call to RefPtr::get() in the generated bindings code

Avoid useless call to RefPtr::get() in the generated bindings code for
null checks.

R=haraken,arv
BUG=

Review URL: https://codereview.chromium.org/26592006

git-svn-id: svn://svn.chromium.org/blink/trunk@159258 bbb929c8-8fbe-4397-9dbb-9b2b20218538