Fix class name which has only space

When length of className is 0, should return ClassStringContent::Empty, this is done by classStringHasClassName(const AtomicString& newClassString. If length >= 1 and only white space, should return ClassStringContent::WhiteSpaceOnly.
Spec is here: https://dom.spec.whatwg.org/#dom-element-classname.
This is from w3c-test: http://w3c-test.org/dom/nodes/Element-classlist.html.

BUG=651744

Test=LayoutTests/imported/wpt/dom/nodes/Element-classlist.html

Review-Url: https://codereview.chromium.org/2402223002
Cr-Commit-Position: refs/heads/master@{#425578}
2 files changed