Let LayoutCustomScrollbarPart inherit LayoutReplaced instead of LayoutBlock

Custom scrollbar parts don't have children. They act more like special
replaced elements. Letting LayoutCustomScrollbarPart inherit
LayoutReplaced can reduce cost.

Change-Id: I496aa9c331485a2ff4a0bbe5c9f2b542abce79ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226924Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774389}

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

@@ -39,7 +39,7 @@ LayoutCustomScrollbarPart::LayoutCustomScrollbarPart(
     ScrollableArea* scrollable_area,
     CustomScrollbar* scrollbar,
     ScrollbarPart part)
-    : LayoutBlock(nullptr),
+    : LayoutReplaced(nullptr, LayoutSize()),
       scrollable_area_(scrollable_area),
       scrollbar_(scrollbar),
       part_(part) {
@@ -211,28 +211,18 @@ void LayoutCustomScrollbarPart::UpdateScrollbarHeight() {
           .Round()));
 }
 
-MinMaxSizes LayoutCustomScrollbarPart::PreferredLogicalWidths() const {
-  return MinMaxSizes();
-}
-
-void LayoutCustomScrollbarPart::StyleWillChange(
-    StyleDifference diff,
-    const ComputedStyle& new_style) {
-  LayoutBlock::StyleWillChange(diff, new_style);
+void LayoutCustomScrollbarPart::UpdateFromStyle() {
+  LayoutReplaced::UpdateFromStyle();
   SetInline(false);
+  ClearPositionedState();
+  SetFloating(false);
 }
 
 void LayoutCustomScrollbarPart::StyleDidChange(StyleDifference diff,
                                                const ComputedStyle* old_style) {
-  LayoutBlock::StyleDidChange(diff, old_style);
-  // See adjustStyleBeforeSet() above.
-  DCHECK(!IsOrthogonalWritingModeRoot());
-  SetInline(false);
-  ClearPositionedState();
-  SetFloating(false);
+  LayoutReplaced::StyleDidChange(diff, old_style);
   if (old_style && (diff.NeedsPaintInvalidation() || diff.NeedsLayout()))
     SetNeedsPaintInvalidation();
-
   RecordPercentLengthStats();
 }
 
@@ -264,7 +254,7 @@ void LayoutCustomScrollbarPart::RecordPercentLengthStats() const {
 void LayoutCustomScrollbarPart::ImageChanged(WrappedImagePtr image,
                                              CanDeferInvalidation defer) {
   SetNeedsPaintInvalidation();
-  LayoutBlock::ImageChanged(image, defer);
+  LayoutReplaced::ImageChanged(image, defer);
 }
 
 void LayoutCustomScrollbarPart::SetNeedsPaintInvalidation() {

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

@@ -26,7 +26,7 @@
 #ifndef THIRD_PARTY_BLINK_RENDERER_CORE_LAYOUT_LAYOUT_CUSTOM_SCROLLBAR_PART_H_
 #define THIRD_PARTY_BLINK_RENDERER_CORE_LAYOUT_LAYOUT_CUSTOM_SCROLLBAR_PART_H_
 
-#include "third_party/blink/renderer/core/layout/layout_block.h"
+#include "third_party/blink/renderer/core/layout/layout_replaced.h"
 #include "third_party/blink/renderer/core/scroll/scroll_types.h"
 
 namespace blink {
@@ -34,7 +34,7 @@ namespace blink {
 class CustomScrollbar;
 class ScrollableArea;
 
-class LayoutCustomScrollbarPart final : public LayoutBlock {
+class LayoutCustomScrollbarPart final : public LayoutReplaced {
  public:
   static LayoutCustomScrollbarPart* CreateAnonymous(Document*,
                                                     ScrollableArea*,
@@ -52,38 +52,34 @@ class LayoutCustomScrollbarPart final : public LayoutBlock {
 
   // Scrollbar parts needs to be rendered at device pixel boundaries.
   LayoutUnit MarginTop() const override {
-    DCHECK(IsIntegerValue(LayoutBlock::MarginTop()));
-    return LayoutBlock::MarginTop();
+    DCHECK(IsIntegerValue(LayoutReplaced::MarginTop()));
+    return LayoutReplaced::MarginTop();
   }
   LayoutUnit MarginBottom() const override {
-    DCHECK(IsIntegerValue(LayoutBlock::MarginBottom()));
-    return LayoutBlock::MarginBottom();
+    DCHECK(IsIntegerValue(LayoutReplaced::MarginBottom()));
+    return LayoutReplaced::MarginBottom();
   }
   LayoutUnit MarginLeft() const override {
-    DCHECK(IsIntegerValue(LayoutBlock::MarginLeft()));
-    return LayoutBlock::MarginLeft();
+    DCHECK(IsIntegerValue(LayoutReplaced::MarginLeft()));
+    return LayoutReplaced::MarginLeft();
   }
   LayoutUnit MarginRight() const override {
-    DCHECK(IsIntegerValue(LayoutBlock::MarginRight()));
-    return LayoutBlock::MarginRight();
+    DCHECK(IsIntegerValue(LayoutReplaced::MarginRight()));
+    return LayoutReplaced::MarginRight();
   }
 
   bool IsOfType(LayoutObjectType type) const override {
     return type == kLayoutObjectLayoutCustomScrollbarPart ||
-           LayoutBlock::IsOfType(type);
+           LayoutReplaced::IsOfType(type);
   }
   ScrollableArea* GetScrollableArea() const { return scrollable_area_; }
 
- protected:
-  void StyleWillChange(StyleDifference,
-                       const ComputedStyle& new_style) override;
-  void StyleDidChange(StyleDifference, const ComputedStyle* old_style) override;
-  void ImageChanged(WrappedImagePtr, CanDeferInvalidation) override;
-
  private:
   LayoutCustomScrollbarPart(ScrollableArea*, CustomScrollbar*, ScrollbarPart);
 
-  MinMaxSizes PreferredLogicalWidths() const override;
+  void UpdateFromStyle() override;
+  void StyleDidChange(StyleDifference, const ComputedStyle* old_style) override;
+  void ImageChanged(WrappedImagePtr, CanDeferInvalidation) override;
 
   // Have all padding getters return 0. The important point here is to avoid
   // resolving percents against the containing block, since scroll bar corners
@@ -103,8 +99,6 @@ class LayoutCustomScrollbarPart final : public LayoutBlock {
 
   void SetNeedsPaintInvalidation();
 
-  bool AllowsOverflowClip() const override { return false; }
-
   void RecordPercentLengthStats() const;
 
   UntracedMember<ScrollableArea> scrollable_area_;