Document.title getter should return text of title element and setter should stop when head element is null

SPEC: http://www.whatwg.org/specs/web-apps/current-work/#document.title

This CL allowed implicit title set from Document::setTitleElement()
and stopped operation on setter when head element is null as per specification.

In addition, with this change, failed test cases will be passed in
http://w3c-test.org/html/dom/documents/dom-tree-accessors/document.title-01.html

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

Plus, this CL deleted obsolete title argument from Document::setTitleElement.

TEST=LayoutTests/fast/dom/Document/title-01.html
    ,LayoutTests/svg/custom/multiple-title-elements.svg

Review URL: https://codereview.chromium.org/384413003

git-svn-id: svn://svn.chromium.org/blink/trunk@178889 bbb929c8-8fbe-4397-9dbb-9b2b20218538