Inline static Create() factory functions from renderer/core/layout

This CL removes Create() factory functions in
//third_party/blink/renderer/core/layout only when it
meets the policy of removing Create() so that we support
only one of Create() methods or public constructors in a class.
Then, this CL makes all callers use MakeGarbageCollected<Foo>
instead.

This CL removes below Create() functions,
  - CustomScrollbar::CreateCustomScrollbar

Bug: 939691
Change-Id: I8d36d02e45569591cf40aaa90b8206090b704b7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1971032Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#725434}

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

@@ -36,14 +36,6 @@
 
 namespace blink {
 
-Scrollbar* CustomScrollbar::CreateCustomScrollbar(
-    ScrollableArea* scrollable_area,
-    ScrollbarOrientation orientation,
-    Element* style_source) {
-  return MakeGarbageCollected<CustomScrollbar>(scrollable_area, orientation,
-                                               style_source);
-}
-
 CustomScrollbar::CustomScrollbar(ScrollableArea* scrollable_area,
                                  ScrollbarOrientation orientation,
                                  Element* style_source)

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

@@ -45,10 +45,6 @@ class LayoutObject;
 // LayoutCustomScrollbarPart.
 class CustomScrollbar final : public Scrollbar {
  public:
-  static Scrollbar* CreateCustomScrollbar(ScrollableArea*,
-                                          ScrollbarOrientation,
-                                          Element*);
-
   CustomScrollbar(ScrollableArea*, ScrollbarOrientation, Element*);
   ~CustomScrollbar() override;
 

third_party/blink/renderer/core/paint/paint_layer_scrollable_area.cc

@@ -99,6 +99,7 @@
 #include "third_party/blink/renderer/core/scroll/smooth_scroll_sequencer.h"
 #include "third_party/blink/renderer/platform/graphics/graphics_layer.h"
 #include "third_party/blink/renderer/platform/graphics/paint/drawing_recorder.h"
+#include "third_party/blink/renderer/platform/heap/heap.h"
 
 namespace blink {
 
@@ -2575,7 +2576,7 @@ Scrollbar* PaintLayerScrollableArea::ScrollbarManager::CreateScrollbar(
       style_source.StyleRef().HasPseudoElementStyle(kPseudoIdScrollbar);
   if (has_custom_scrollbar_style) {
     DCHECK(style_source.GetNode() && style_source.GetNode()->IsElementNode());
-    scrollbar = CustomScrollbar::CreateCustomScrollbar(
+    scrollbar = MakeGarbageCollected<CustomScrollbar>(
         ScrollableArea(), orientation, To<Element>(style_source.GetNode()));
   } else {
     ScrollbarControlSize scrollbar_size = kRegularScrollbar;