Remove unnecessary JSC::Handle null checks in bindings code.

Remove unnecessary JSC::Handle null checks in bindings code.
<https://webkit.org/b/130356>

Use Handle::slot() directly instead of going through Handle::get().
This avoids null checking the HandleSlot, which isn't necessary here
anyway, and the code already assumes it'll never be null.

Reviewed by Gavin Barraclough.

* bindings/js/JSCSSRuleListCustom.cpp:
(WebCore::JSCSSRuleListOwner::isReachableFromOpaqueRoots):
* bindings/js/JSCSSValueCustom.cpp:
(WebCore::JSCSSValueOwner::isReachableFromOpaqueRoots):
(WebCore::JSCSSValueOwner::finalize):
* bindings/js/JSMutationObserverCustom.cpp:
(WebCore::JSMutationObserverOwner::isReachableFromOpaqueRoots):
* bindings/js/JSNodeCustom.cpp:
(WebCore::JSNodeOwner::isReachableFromOpaqueRoots):
* bindings/js/JSNodeListCustom.cpp:
(WebCore::JSNodeListOwner::isReachableFromOpaqueRoots):
* bindings/js/JSTextTrackCueCustom.cpp:
(WebCore::JSTextTrackCueOwner::isReachableFromOpaqueRoots):
* bindings/js/WebCoreTypedArrayController.cpp:
(WebCore::WebCoreTypedArrayController::JSArrayBufferOwner::isReachableFromOpaqueRoots):
(WebCore::WebCoreTypedArrayController::JSArrayBufferOwner::finalize):
* bindings/scripts/CodeGeneratorJS.pm:
(GenerateImplementation):
* bridge/runtime_root.cpp:
(JSC::Bindings::RootObject::finalize):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165757 268f45cc-cd09-0410-ab3c-d52691b4dbfc