Range.deleteContents shouldn't throw HierarchyRequestError on doctype

SPEC: http://dom.spec.whatwg.org/#dom-range-deletecontents

Unlike extractContents, deleteContents doesn't throw error.
So this CL modified implementation as per specified.

Behavior in other browsers.
*)FF: PASS
*)IE: PASS

In addition 4 failed cases will be passed with this CL in
http://w3c-test.org/dom/ranges/Range-deleteContents.html

TEST=LayoutTests/fast/dom/Range/deleteContents-doctype.html

Review URL: https://codereview.chromium.org/451403002

git-svn-id: svn://svn.chromium.org/blink/trunk@180143 bbb929c8-8fbe-4397-9dbb-9b2b20218538