Remove dependencies from ScriptValue on SerializedScriptValue

This removes the only direct dependencies from ScriptValue onto core.

Also removed unneeded includes from ScriptValue.h.

R=abarth

Review URL: https://codereview.chromium.org/55293002

git-svn-id: svn://svn.chromium.org/blink/trunk@161281 bbb929c8-8fbe-4397-9dbb-9b2b20218538