Introduce toSVGSMILElement, and use it

Source/core/svg/animation/SVGSMILElement.cpp

Index: Source/core/svg/animation/SVGSMILElement.cpp
diff --git a/Source/core/svg/animation/SVGSMILElement.cpp b/Source/core/svg/animation/SVGSMILElement.cpp
index b84205a95ed9eb8f7a5adc1f246e7e84a17ed50e..f28173bb35b412eb580bd968e2fcd6225a3d30e3 100644
--- a/Source/core/svg/animation/SVGSMILElement.cpp
+++ b/Source/core/svg/animation/SVGSMILElement.cpp
@@ -536,8 +536,7 @@ void SVGSMILElement::connectConditions()
                 condition.m_syncbase = 0;
                 continue;
             }
-            SVGSMILElement* syncbase = static_cast<SVGSMILElement*>(condition.m_syncbase.get());
-            syncbase->addTimeDependent(this);
+            toSVGSMILElement(condition.m_syncbase.get())->addTimeDependent(this);
         }
     }
 }
@@ -564,10 +563,8 @@ void SVGSMILElement::disconnectConditions()
             condition.m_eventListener->disconnectAnimation();
             condition.m_eventListener = 0;
         } else if (condition.m_type == Condition::Syncbase) {
-            if (condition.m_syncbase) {
-                ASSERT(isSMILElement(condition.m_syncbase.get()));
-                static_cast<SVGSMILElement*>(condition.m_syncbase.get())->removeTimeDependent(this);
-            }
+            if (condition.m_syncbase)
+                toSVGSMILElement(condition.m_syncbase.get())->removeTimeDependent(this);
         }
         condition.m_syncbase = 0;
     }