Remove mutation foot-gun from CSSPrimitiveValue

Misuse of this method could easily cause
memory leaks and UAFs. It wasn't actually doing anything
in the one place it was called, so I removed it.

I also fixed another FIXME about how unsigned/signed
constructors are confusing.  At least now we'll ASSERT
if you got the wrong one.

Review URL: https://codereview.chromium.org/209353004

git-svn-id: svn://svn.chromium.org/blink/trunk@170029 bbb929c8-8fbe-4397-9dbb-9b2b20218538