Use new downcast helper for blink::SVGFEImageElement

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

Bug: 891908
Change-Id: Ie98c73604c13c6e3171ae1b050453326b8b1ec1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936582Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720312}

third_party/blink/renderer/core/svg/graphics/svg_image.cc

@@ -160,7 +160,7 @@ bool SVGImage::CurrentFrameHasSingleSecurityOrigin() const {
     if (auto* image = ToSVGImageElementOrNull(*node)) {
       if (!image->CurrentFrameHasSingleSecurityOrigin())
         return false;
-    } else if (auto* fe_image = ToSVGFEImageElementOrNull(*node)) {
+    } else if (auto* fe_image = DynamicTo<SVGFEImageElement>(*node)) {
       if (!fe_image->CurrentFrameHasSingleSecurityOrigin())
         return false;
     }