Add assertions for CSSPrimitiveValue's m_value.valueID accessor.

This is a backport from WebKit r153401 from Sam Weinig reviewed by
Andreas Kling.

Review URL: https://chromiumcodereview.appspot.com/21364004

git-svn-id: svn://svn.chromium.org/blink/trunk@155274 bbb929c8-8fbe-4397-9dbb-9b2b20218538