Cleanup [SetWrapperReferenceFromReference] cruft

Followup to:
Kill [SetWrapperReferenceFromReference] IDL extended attribute
https://codereview.chromium.org/181513002/

TBR=haraken
BUG=345503

Review URL: https://codereview.chromium.org/176993007

git-svn-id: svn://svn.chromium.org/blink/trunk@168259 bbb929c8-8fbe-4397-9dbb-9b2b20218538