While RetainPtr is not necessary under ARC, having it available makes it easier to transition
existing code from manual retain / release to ARC.

Under ARC, the object member of RetainPtr is treated as a strong reference by the compiler.
This means that merely assigning to the member variable is sufficient to retain the object,
and clearing the member variable is sufficient to release it. We still need to explicitly
CFRetain / CFRelease CoreFoundation types so the explicit calls to these functions are
moved in to helper functions and overloading is used to have the Objective-C object versions
of them be no-ops under ARC.

Reviewed by Anders Carlsson.

* wtf/RetainPtr.h:
(WTF::retain): Continue to always CFRetain / CFRelease CoreFoundation objects. Only CFRetain / CFRelease
Objective-C objects when using manual retain / release.
(WTF::release): Ditto.
(WTF::adoptNSReference): Adopting references will be handled automatically by the compiler
when possible under ARC by eliminating redundant retain / release pairs.
(WTF::RetainPtr::ImplicitConversionToBoolIsNotAllowed): A new method that exists only to be used by the
conversion to the unspecified bool type.
(WTF::RetainPtr::operator UnspecifiedBoolType): Switch to using a pointer to a member function as the
unspecified bool type to avoid warnings from the compiler when casting Objective-C object types under ARC.

(WTF::RetainPtr::RetainPtr): Switch to our retain / release helper functions.
(WTF::RetainPtr::~RetainPtr): Ditto.
(WTF::::RetainPtr): Ditto.
(WTF::::clear): Ditto.
(WTF::=): Ditto.
(WTF::adoptCF): Annotate the argument with CF_RELEASES_ARGUMENT on both the declaration and the definition.
(WTF::adoptNS): Ditto for NS_RELEASES_ARGUMENT.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153637 268f45cc-cd09-0410-ab3c-d52691b4dbfc