Add assertions for CSSPrimitiveValue's m_value.valueID accessor.

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

In addition of the backport the patch fixes a problem where a new
ASSERT was triggered. The text-align property can now take
<string> as a value (see http://www.w3.org/TR/css3-text/#text-align0)
and the parsing code seems to be partially implemented but the rendering
code does not yet take into account this case. The StyleBuilder was
asserting (thanks to the new ASSERTS) because we were trying to use
a CSSPrimitiveValue holding a string as one holding an ident. Needless
to say that the previous code was setting a buggy ETextAlign.

Review URL: https://chromiumcodereview.appspot.com/20722005

git-svn-id: svn://svn.chromium.org/blink/trunk@155348 bbb929c8-8fbe-4397-9dbb-9b2b20218538