Simplify DOMTokenList::validateToken() code

Simplify DOMTokenList::validateToken() code by calling WTFString::find()
instead of manually iterating through the String to find HTML spaces.

R=rob.buis@samsung.com

Review URL: https://codereview.chromium.org/479243002

git-svn-id: svn://svn.chromium.org/blink/trunk@180486 bbb929c8-8fbe-4397-9dbb-9b2b20218538