Use new downcast helper for blink::SVGSetElement

This CL has goal to use IsA<SVGSetElement>(element) in place of
IsSVGSetElement(element)

Bug: 891908
Change-Id: I88613d67d85f96821fc70441cc24e341b70ce19b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936620Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720496}

third_party/blink/renderer/core/svg/animation/svg_smil_element.cc

@@ -1005,7 +1005,7 @@ SMILTime SVGSMILElement::NextProgressTime(SMILTime presentation_time) const {
     // If duration is indefinite the value does not actually change over time.
     // Same is true for <set>.
     SMILTime simple_duration = SimpleDuration();
-    if (simple_duration.IsIndefinite() || IsSVGSetElement(*this)) {
+    if (simple_duration.IsIndefinite() || IsA<SVGSetElement>(*this)) {
       SMILTime repeating_duration_end = interval_.begin + RepeatingDuration();
       // We are supposed to do freeze semantics when repeating ends, even if the
       // element is still active.

third_party/blink/renderer/core/svg/svg_animate_element.cc

@@ -351,7 +351,7 @@ void SVGAnimateElement::CalculateAnimatedValue(
   DCHECK_EQ(result_animation_element->GetAnimatedPropertyType(),
             GetAnimatedPropertyType());
 
-  if (IsSVGSetElement(*this))
+  if (IsA<SVGSetElement>(*this))
     percentage = 1;
 
   if (GetCalcMode() == kCalcModeDiscrete)
@@ -420,7 +420,7 @@ bool SVGAnimateElement::CalculateFromAndByValues(const String& from_string,
       !AnimatedPropertyTypeSupportsAddition())
     return false;
 
-  DCHECK(!IsSVGSetElement(*this));
+  DCHECK(!IsA<SVGSetElement>(*this));
 
   from_property_ = CreatePropertyForAnimation(from_string);
   from_property_value_type_ = PropertyValueType(AttributeName(), from_string);