TextControl NG: Remove DowncastTraits<LayoutTextControlSingleLine>

Text field inputs in LayoutNG might not use a subclass of
LayoutTextControlSingleLine. We should avoid operations on it.

LayoutTextControlSingleLineTest doesn't need to use
LayoutTextControlSingleLine, and LayoutBox is enough.

This CL has no behavior changes.

Bug: 1040826
Change-Id: Ica5a0acbf532b5a781b515d0ce9de1c4262abac0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437721
Commit-Queue: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811961}

third_party/blink/renderer/core/layout/layout_box.h

@@ -2335,6 +2335,11 @@ class CORE_EXPORT LayoutBox : public LayoutBoxModelObject {
   NGLayoutResultList layout_results_;
 };
 
+template <>
+struct DowncastTraits<LayoutBox> {
+  static bool AllowFrom(const LayoutObject& object) { return object.IsBox(); }
+};
+
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBox, IsBox());
 
 inline LayoutBox* LayoutBox::PreviousSiblingBox() const {

third_party/blink/renderer/core/layout/layout_text_control_single_line.h

@@ -86,13 +86,6 @@ class LayoutTextControlSingleLine : public LayoutTextControl {
   HTMLElement* InnerSpinButtonElement() const;
 };
 
-template <>
-struct DowncastTraits<LayoutTextControlSingleLine> {
-  static bool AllowFrom(const LayoutObject& object) {
-    return object.IsTextField();
-  }
-};
-
 // ----------------------------
 
 class LayoutTextControlInnerEditor : public LayoutBlockFlow {

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

@@ -23,8 +23,7 @@ TEST_F(LayoutTextControlSingleLineTest, VisualOverflowCleared) {
     </style>
     <input id=input type="text"></input.
   )HTML");
-  auto* input =
-      To<LayoutTextControlSingleLine>(GetLayoutObjectByElementId("input"));
+  auto* input = To<LayoutBox>(GetLayoutObjectByElementId("input"));
   if (::features::IsFormControlsRefreshEnabled()) {
     EXPECT_EQ(LayoutRect(-3, -3, 74, 72), input->SelfVisualOverflowRect());
   } else {