Use new downcast helper for blink::SVGPatternElement

This CL has goal to use To<SVGPatternElement> and
DynamicTo<SVGPatternElement> as new downcast helper

Bug: 891908
Change-Id: Ibcc937ce3e849c5132d7e412246eb32268e16f87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936614Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720118}

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

@@ -145,7 +145,7 @@ SVGPaintServer LayoutSVGResourcePattern::PreparePaintServer(
   // no side-effects though.
   if (should_collect_pattern_attributes_) {
     attributes_wrapper_->Set(PatternAttributes());
-    SVGPatternElement* pattern_element = ToSVGPatternElement(GetElement());
+    auto* pattern_element = To<SVGPatternElement>(GetElement());
     pattern_element->CollectPatternAttributes(MutableAttributes());
     should_collect_pattern_attributes_ = false;
   }

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

@@ -593,7 +593,7 @@ void WriteSVGResourceContainer(WTF::TextStream& ts,
     // patterns using xlink:href, we need to build the full inheritance chain,
     // aka. collectPatternProperties()
     PatternAttributes attributes;
-    ToSVGPatternElement(pattern->GetElement())
+    To<SVGPatternElement>(pattern->GetElement())
         ->CollectPatternAttributes(attributes);
 
     WriteNameValuePair(ts, "patternUnits", attributes.PatternUnits());

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

@@ -232,7 +232,7 @@ std::unique_ptr<SVGResources> SVGResources::BuildResources(
     }
   }
 
-  if (auto* pattern = ToSVGPatternElementOrNull(element)) {
+  if (auto* pattern = DynamicTo<SVGPatternElement>(element)) {
     const SVGPatternElement* directly_referenced_pattern =
         pattern->ReferencedElement();
     if (directly_referenced_pattern) {

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

@@ -250,7 +250,8 @@ static void SetPatternAttributes(const SVGPatternElement& element,
 }
 
 const SVGPatternElement* SVGPatternElement::ReferencedElement() const {
-  return ToSVGPatternElementOrNull(resource_ ? resource_->Target() : nullptr);
+  return DynamicTo<SVGPatternElement>(resource_ ? resource_->Target()
+                                                : nullptr);
 }
 
 void SVGPatternElement::CollectPatternAttributes(