Use new downcast helper for blink::SVGMaskElement

This CL has two goals,
  1. Use To<SVGMaskElement> as new downcast helper
  2. Use IsA<SVGMaskElement>(element) in place of
     IsSVGMaskElement(element)

Bug: 891908
Change-Id: I2781f717a10afb6187228647581b99a864ad851e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936612Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720432}

third_party/blink/renderer/core/layout/svg/layout_svg_resource_masker.cc

@@ -96,14 +96,14 @@ void LayoutSVGResourceMasker::CalculateMaskContentVisualRect() {
 }
 
 SVGUnitTypes::SVGUnitType LayoutSVGResourceMasker::MaskUnits() const {
-  return ToSVGMaskElement(GetElement())
+  return To<SVGMaskElement>(GetElement())
       ->maskUnits()
       ->CurrentValue()
       ->EnumValue();
 }
 
 SVGUnitTypes::SVGUnitType LayoutSVGResourceMasker::MaskContentUnits() const {
-  return ToSVGMaskElement(GetElement())
+  return To<SVGMaskElement>(GetElement())
       ->maskContentUnits()
       ->CurrentValue()
       ->EnumValue();
@@ -111,7 +111,7 @@ SVGUnitTypes::SVGUnitType LayoutSVGResourceMasker::MaskContentUnits() const {
 
 FloatRect LayoutSVGResourceMasker::ResourceBoundingBox(
     const FloatRect& reference_box) {
-  SVGMaskElement* mask_element = ToSVGMaskElement(GetElement());
+  auto* mask_element = To<SVGMaskElement>(GetElement());
   DCHECK(mask_element);
 
   FloatRect mask_boundaries = SVGLengthContext::ResolveRectangle(

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

@@ -51,7 +51,7 @@ bool TargetCanHaveMotionTransform(const SVGElement& target) {
          IsA<SVGEllipseElement>(target) || IsA<SVGLineElement>(target) ||
          IsA<SVGPolylineElement>(target) || IsA<SVGPolygonElement>(target) ||
          IsA<SVGTextElement>(target) || IsSVGClipPathElement(target) ||
-         IsSVGMaskElement(target) || IsSVGAElement(target) ||
+         IsA<SVGMaskElement>(target) || IsSVGAElement(target) ||
          IsSVGForeignObjectElement(target);
 }
 }