Use new downcast helper for blink::HTMLSourceElement

This primary goal of this CL is to use DynamicTo<HTMLSlotElement> as
new downcast helper

Bug: 891908
Change-Id: Iad3033d44acf245e16c50f0a18cd196cf5791566
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874415
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708970}

third_party/blink/renderer/core/html/html_image_element.cc

@@ -327,10 +327,10 @@ ImageCandidate HTMLImageElement::FindBestFitImageFromPictureParent() {
     if (child == this)
       return ImageCandidate();
 
-    if (!IsHTMLSourceElement(*child))
+    auto* source = DynamicTo<HTMLSourceElement>(child);
+    if (!source)
       continue;
 
-    HTMLSourceElement* source = ToHTMLSourceElement(child);
     if (!source->FastGetAttribute(html_names::kSrcAttr).IsNull()) {
       Deprecation::CountDeprecation(GetDocument(),
                                     WebFeature::kPictureSourceSrc);

third_party/blink/renderer/core/html/media/html_media_element.cc

@@ -3084,12 +3084,9 @@ KURL HTMLMediaElement::SelectNextSourceChild(
       continue;
     looking_for_start_node = false;
 
-    if (!IsHTMLSourceElement(*node))
+    source = DynamicTo<HTMLSourceElement>(node);
+    if (!source || node->parentNode() != this)
       continue;
-    if (node->parentNode() != this)
-      continue;
-
-    source = ToHTMLSourceElement(node);
 
     // 2. If candidate does not have a src attribute, or if its src
     // attribute's value is the empty string ... jump down to the failed