[mathml] Minor cleanup of LayoutNGMathMLBlock

* Forward declare MathMLElement instead of including the header.
* Use new isA<MathMLElement> cast syntax.

Bug: 6606
Change-Id: I5f8d95668849dc76f2bf05c8c4efb72004466310
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153146Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/master@{#759799}

third_party/blink/renderer/core/layout/ng/mathml/layout_ng_mathml_block.cc

@@ -6,6 +6,7 @@
 
 #include "third_party/blink/renderer/core/layout/layout_analyzer.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_layout_result.h"
+#include "third_party/blink/renderer/core/mathml/mathml_element.h"
 
 namespace blink {
 
@@ -34,7 +35,7 @@ bool LayoutNGMathMLBlock::IsOfType(LayoutObjectType type) const {
 
 bool LayoutNGMathMLBlock::IsChildAllowed(LayoutObject* child,
                                          const ComputedStyle&) const {
-  return child->GetNode() && child->GetNode()->IsMathMLElement();
+  return child->GetNode() && IsA<MathMLElement>(child->GetNode());
 }
 
 bool LayoutNGMathMLBlock::CanHaveChildren() const {

third_party/blink/renderer/core/layout/ng/mathml/layout_ng_mathml_block.h

@@ -6,10 +6,11 @@
 #define THIRD_PARTY_BLINK_RENDERER_CORE_LAYOUT_NG_MATHML_LAYOUT_NG_MATHML_BLOCK_H_
 
 #include "third_party/blink/renderer/core/layout/ng/layout_ng_mixin.h"
-#include "third_party/blink/renderer/core/mathml/mathml_element.h"
 
 namespace blink {
 
+class MathMLElement;
+
 class LayoutNGMathMLBlock : public LayoutNGMixin<LayoutBlock> {
  public:
   explicit LayoutNGMathMLBlock(MathMLElement*);