Slider NG: Add blink::SliderTrackElement

This is a preparation to attach LayoutSliderContainer to the slider
track part, instead of the slider container part. LayoutSliderContainer
adjusts the position of the thumb part, but the thumb part is not a
child of the slider container part, and it's a child of the slider
track part.

This CL has no behavior changes.

Bug: 1040826
Change-Id: Ie520e4f9fe5c474c40b55eb0e2d5d308c05300cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391893
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@{#804315}

third_party/blink/renderer/core/html/BUILD.gn

@@ -252,6 +252,8 @@ blink_core_sources("html") {
     "forms/select_type.h",
     "forms/slider_thumb_element.cc",
     "forms/slider_thumb_element.h",
+    "forms/slider_track_element.cc",
+    "forms/slider_track_element.h",
     "forms/spin_button_element.cc",
     "forms/spin_button_element.h",
     "forms/step_range.cc",

third_party/blink/renderer/core/html/forms/range_input_type.cc

@@ -46,8 +46,8 @@
 #include "third_party/blink/renderer/core/html/forms/html_input_element.h"
 #include "third_party/blink/renderer/core/html/forms/html_option_element.h"
 #include "third_party/blink/renderer/core/html/forms/slider_thumb_element.h"
+#include "third_party/blink/renderer/core/html/forms/slider_track_element.h"
 #include "third_party/blink/renderer/core/html/forms/step_range.h"
-#include "third_party/blink/renderer/core/html/html_div_element.h"
 #include "third_party/blink/renderer/core/html/parser/html_parser_idioms.h"
 #include "third_party/blink/renderer/core/html/shadow/shadow_element_names.h"
 #include "third_party/blink/renderer/core/html_names.h"
@@ -244,7 +244,7 @@ void RangeInputType::CreateShadowSubtree() {
   DCHECK(IsShadowHost(GetElement()));
 
   Document& document = GetElement().GetDocument();
-  auto* track = MakeGarbageCollected<HTMLDivElement>(document);
+  auto* track = MakeGarbageCollected<blink::SliderTrackElement>(document);
   track->SetShadowPseudoId(AtomicString("-webkit-slider-runnable-track"));
   track->setAttribute(html_names::kIdAttr,
                       shadow_element_names::kIdSliderTrack);

third_party/blink/renderer/core/html/forms/slider_track_element.cc

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "third_party/blink/renderer/core/html/forms/slider_track_element.h"
+
+namespace blink {
+
+SliderTrackElement::SliderTrackElement(Document& document)
+    : HTMLDivElement(document) {}
+
+LayoutObject* SliderTrackElement::CreateLayoutObject(const ComputedStyle& style,
+                                                     LegacyLayout legacy) {
+  // TODO(crbug.com/1040826): Introduce LayoutSliderTrack, and use it here.
+  return HTMLDivElement::CreateLayoutObject(style, legacy);
+}
+
+}  // namespace blink

third_party/blink/renderer/core/html/forms/slider_track_element.h

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef THIRD_PARTY_BLINK_RENDERER_CORE_HTML_FORMS_SLIDER_TRACK_ELEMENT_H_
+#define THIRD_PARTY_BLINK_RENDERER_CORE_HTML_FORMS_SLIDER_TRACK_ELEMENT_H_
+
+#include "third_party/blink/renderer/core/html/html_div_element.h"
+
+namespace blink {
+
+// SliderTrackElement represents a track part of <input type=range>, and it's a
+// parent of a SliderThumbElement. We need this dedicated C++ class because we'd
+// like to provide a dedicated LayoutObject for this.
+class SliderTrackElement final : public HTMLDivElement {
+ public:
+  explicit SliderTrackElement(Document& document);
+
+ private:
+  LayoutObject* CreateLayoutObject(const ComputedStyle& style,
+                                   LegacyLayout legacy) override;
+};
+
+}  // namespace blink
+#endif  // THIRD_PARTY_BLINK_RENDERER_CORE_HTML_FORMS_SLIDER_TRACK_ELEMENT_H_