Make Document::implementation() method return reference

DOMImplementation can not be null and this CL only makes implementation() return
a reference instead of pointer.

Review URL: https://codereview.chromium.org/184813005

git-svn-id: svn://svn.chromium.org/blink/trunk@168436 bbb929c8-8fbe-4397-9dbb-9b2b20218538