Don't support mixed dashed/camel-cased CSS property names

CSSStyleDeclaration's custom getter and setter support property names that
are either dashed ("border-top-width") or camel-cased ("borderTopWidth")
but shouldn't support names that are both (such as "border-topWidth".)

BUG=334562

Review URL: https://codereview.chromium.org/141693002

git-svn-id: svn://svn.chromium.org/blink/trunk@165331 bbb929c8-8fbe-4397-9dbb-9b2b20218538