Objective-C API: Objective-C functions exposed to JavaScript have the wrong type (object instead of function)
https://bugs.webkit.org/show_bug.cgi?id=105892

Reviewed by Geoffrey Garen.

Changed ObjCCallbackFunction to subclass JSCallbackFunction which already has all of the machinery to call
functions using the C API. Since ObjCCallbackFunction is now a JSCell, we changed the old implementation of
ObjCCallbackFunction to be the internal implementation and keep track of all the proper data so that we 
don't have to put all of that in the header, which will now be included from C++ files (e.g. JSGlobalObject.cpp).

* API/JSCallbackFunction.cpp: Change JSCallbackFunction to allow subclassing. Originally it was internally
passing its own Structure up the chain of constructors, but we now want to be able to pass other Structures as well.
(JSC::JSCallbackFunction::JSCallbackFunction):
(JSC::JSCallbackFunction::create):
* API/JSCallbackFunction.h:
(JSCallbackFunction):
* API/JSWrapperMap.mm: Changed interface to tryUnwrapBlock.
(tryUnwrapObjcObject):
* API/ObjCCallbackFunction.h:
(ObjCCallbackFunction): Moved into the JSC namespace, just like JSCallbackFunction.
(JSC::ObjCCallbackFunction::createStructure): Overridden so that the correct ClassInfo gets used since we have 
a destructor.
(JSC::ObjCCallbackFunction::impl): Getter for the internal impl.
* API/ObjCCallbackFunction.mm:
(JSC::ObjCCallbackFunctionImpl::ObjCCallbackFunctionImpl): What used to be ObjCCallbackFunction is now 
ObjCCallbackFunctionImpl. It handles the Objective-C specific parts of managing callback functions.
(JSC::ObjCCallbackFunctionImpl::~ObjCCallbackFunctionImpl):
(JSC::objCCallbackFunctionCallAsFunction): Same as the old one, but now it casts to ObjCCallbackFunction and grabs the impl 
rather than using JSObjectGetPrivate.
(JSC::ObjCCallbackFunction::ObjCCallbackFunction): New bits to allow being part of the JSCell hierarchy.
(JSC::ObjCCallbackFunction::create):
(JSC::ObjCCallbackFunction::destroy):
(JSC::ObjCCallbackFunctionImpl::call): Handles the actual invocation, just like it used to.
(objCCallbackFunctionForInvocation):
(tryUnwrapBlock): Changed to check the ClassInfo for inheritance directly, rather than going through the C API call.
* API/tests/testapi.mm: Added new test to make sure that doing Function.prototype.toString.call(f) won't result in 
an error when f is an Objective-C method or block underneath the covers.
* runtime/JSGlobalObject.cpp: Added new Structure for ObjCCallbackFunction.
(JSC::JSGlobalObject::reset):
(JSC::JSGlobalObject::visitChildren):
* runtime/JSGlobalObject.h:
(JSGlobalObject):
(JSC::JSGlobalObject::objcCallbackFunctionStructure):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145848 268f45cc-cd09-0410-ab3c-d52691b4dbfc