Revert "Add assertions for CSSPrimitiveValue's m_value.valueID accessor."

This reverts commit 7f93f86e32443f28c7a28976fe51b138c25b3016.
Following test actually hits the assertion:
- fast/css/first-letter-anonymous-block-crash.html

TBR=alexis.menard@intel.com
BUG=

Review URL: https://codereview.chromium.org/21464004

git-svn-id: svn://svn.chromium.org/blink/trunk@155308 bbb929c8-8fbe-4397-9dbb-9b2b20218538