[QtScript/JavaScriptCore] Make native setter in prototype work

Patch: http://gitorious.org/+qtwebkit-developers/qtwebkit/qtwebkit-qtscript-integration/commit/0c9d8ad1923bcc38369a63bd5013233c660ea520

Reason: Without this patch, QTBUG-5749 results.

This patch is likely to have performance impact.
First of all, a testcase that uses the public JSC C API to reproduce this issue is needed. In JSClassDefinition, the "staticValues" field is relevant (are those properties put in the prototype or the instance?).