Don't check if SerializedScriptValue's Reader::m_version is less than 0

Don't check if SerializedScriptValue's Reader::m_version is less than 0 as the
variable is unsigned.

R=haraken@chromium.org

Review URL: https://codereview.chromium.org/238273010

git-svn-id: svn://svn.chromium.org/blink/trunk@171827 bbb929c8-8fbe-4397-9dbb-9b2b20218538