Correct comment references to DocumentLoader::replaceDocumentWhileExecutingJavascriptURL

DocumentWriter::replaceDocumentWhileExecutingJavascriptURL is now moved
DocumentLoader. Correct its references and remove unused decl.

BUG=421289

Review URL: https://codereview.chromium.org/639263002

git-svn-id: svn://svn.chromium.org/blink/trunk@183443 bbb929c8-8fbe-4397-9dbb-9b2b20218538