Use new downcast helper for blink::HTMLFrameElement

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

Bug: 891908
Change-Id: Id895e0cf6c250307fca10a2bf4b40228b98ba889
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1741444
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685128}

third_party/blink/renderer/core/css/resolver/style_adjuster.cc

@@ -229,7 +229,7 @@ static void AdjustStyleForHTMLElement(ComputedStyle& style,
     return;
   }
 
-  if (IsHTMLFrameElement(element) || IsHTMLFrameSetElement(element)) {
+  if (IsA<HTMLFrameElement>(element) || IsHTMLFrameSetElement(element)) {
     // Frames and framesets never honor position:relative or position:absolute.
     // This is necessary to fix a crash where a site tries to position these
     // objects. They also never honor display nor floating.

third_party/blink/renderer/core/html/html_frame_element_base.h

@@ -88,7 +88,7 @@ class CORE_EXPORT HTMLFrameElementBase : public HTMLFrameOwnerElement {
 };
 
 inline bool IsHTMLFrameElementBase(const HTMLElement& element) {
-  return IsHTMLFrameElement(element) || IsHTMLIFrameElement(element);
+  return IsA<HTMLFrameElement>(element) || IsHTMLIFrameElement(element);
 }
 
 DEFINE_HTMLELEMENT_TYPE_CASTS_WITH_FUNCTION(HTMLFrameElementBase);

third_party/blink/renderer/core/layout/layout_frame.cc

@@ -37,7 +37,7 @@ LayoutFrame::LayoutFrame(HTMLFrameElement* frame)
 }
 
 FrameEdgeInfo LayoutFrame::EdgeInfo() const {
-  HTMLFrameElement* element = ToHTMLFrameElement(GetNode());
+  auto* element = To<HTMLFrameElement>(GetNode());
   return FrameEdgeInfo(element->NoResize(), element->HasFrameBorder());
 }