Have DOMImplementation take the Document argument by reference

Have DOMImplementation take the Document argument by reference as its
implementation expected the Document pointer to never be null. This
makes the code look safer.

R=adamk

Review URL: https://codereview.chromium.org/65043011

git-svn-id: svn://svn.chromium.org/blink/trunk@162126 bbb929c8-8fbe-4397-9dbb-9b2b20218538