Use type cast macro for AnimatableLengthPoint|3D instead of manual toFoo()

Source/core/accessibility/AXSpinButton.cpp

Index: Source/core/accessibility/AXSpinButton.cpp
diff --git a/Source/core/accessibility/AXSpinButton.cpp b/Source/core/accessibility/AXSpinButton.cpp
index e73482ed419f10d35c4d856b3bf27543a29c605d..475eefbecd7823c260db879616527c82258336fa 100644
--- a/Source/core/accessibility/AXSpinButton.cpp
+++ b/Source/core/accessibility/AXSpinButton.cpp
@@ -62,12 +62,12 @@ void AXSpinButton::addChildren()
 {
     m_haveChildren = true;
 
-    AXSpinButtonPart* incrementor = static_cast<AXSpinButtonPart*>(axObjectCache()->getOrCreate(SpinButtonPartRole));
+    AXSpinButtonPart* incrementor = toAXSpinButtonPart(axObjectCache()->getOrCreate(SpinButtonPartRole));
     incrementor->setIsIncrementor(true);
     incrementor->setParent(this);
     m_children.append(incrementor);
 
-    AXSpinButtonPart* decrementor = static_cast<AXSpinButtonPart*>(axObjectCache()->getOrCreate(SpinButtonPartRole));
+    AXSpinButtonPart* decrementor = toAXSpinButtonPart(axObjectCache()->getOrCreate(SpinButtonPartRole));
     decrementor->setIsIncrementor(false);
     decrementor->setParent(this);
     m_children.append(decrementor);