Use new downcast helper for blink::SVGLinearGradientElement

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

Bug: 891908
Change-Id: I3f775ab96a3fab152b797aa29d9f7db3ae962b10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936609Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#719595}

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

@@ -36,7 +36,7 @@ LayoutSVGResourceLinearGradient::~LayoutSVGResourceLinearGradient() = default;
 bool LayoutSVGResourceLinearGradient::CollectGradientAttributes() {
   DCHECK(GetElement());
   attributes_wrapper_->Set(LinearGradientAttributes());
-  return ToSVGLinearGradientElement(GetElement())
+  return To<SVGLinearGradientElement>(GetElement())
       ->CollectGradientAttributes(MutableAttributes());
 }
 

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

@@ -614,7 +614,7 @@ void WriteSVGResourceContainer(WTF::TextStream& ts,
     // gradients using xlink:href, we need to build the full inheritance chain,
     // aka. collectGradientProperties()
     LinearGradientAttributes attributes;
-    ToSVGLinearGradientElement(gradient->GetElement())
+    To<SVGLinearGradientElement>(gradient->GetElement())
         ->CollectGradientAttributes(attributes);
     WriteCommonGradientProperties(ts, attributes);
 

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

@@ -96,7 +96,7 @@ static void SetGradientAttributes(const SVGGradientElement& element,
 
   if (!is_linear)
     return;
-  const SVGLinearGradientElement& linear = ToSVGLinearGradientElement(element);
+  const auto& linear = To<SVGLinearGradientElement>(element);
 
   if (!attributes.HasX1() && linear.x1()->IsSpecified())
     attributes.SetX1(linear.x1()->CurrentValue());
@@ -120,7 +120,7 @@ bool SVGLinearGradientElement::CollectGradientAttributes(
 
   while (true) {
     SetGradientAttributes(*current, attributes,
-                          IsSVGLinearGradientElement(*current));
+                          IsA<SVGLinearGradientElement>(*current));
     visited.insert(current);
 
     current = current->ReferencedElement();