Get rid of custom code for HTMLDocument.write() / writeln()

Get rid of custom code for HTMLDocument.write() / writeln() by making the
method variadic in Web IDL. Although this is not according to specification,
this matches the behavior of our custom code and the one of FF / IE.

R=haraken
TEST=fast/dom/HTMLDocument/document-write-variadic.html

Review URL: https://codereview.chromium.org/59693007

git-svn-id: svn://svn.chromium.org/blink/trunk@161547 bbb929c8-8fbe-4397-9dbb-9b2b20218538