Use new downcast helper for blink::SVGMPathElement

This CL has goal to use DynamicTo<SVGMPathElement> as new downcast
helper

Bug: 891908
Change-Id: I65766f82e3d2834b5bad1c76397380cdfee12479
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936610Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#719676}

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

@@ -117,7 +117,7 @@ void SVGPathElement::InvalidateMPathDependencies() {
   // dependencies manually.
   if (SVGElementSet* dependencies = SetOfIncomingReferences()) {
     for (SVGElement* element : *dependencies) {
-      if (auto* mpath = ToSVGMPathElementOrNull(*element))
+      if (auto* mpath = DynamicTo<SVGMPathElement>(*element))
         mpath->TargetPathChanged();
     }
   }