<rdar://problem/4634874> WebScriptObject and WebUndefined are no longer defined by WebKit
Moves WebScriptObject and WebUndefined up to WebCore.
This change does create an upwards-dependancy on WebScriptObject existing
in the loaded process, but this code path in JavaScriptCore does not get used
unless it is through WebKit/WebCore. Moving all of the binding code out of JavaScriptCore might make sense in the future.