Remove pre-finalizer from from Scrollbar.

The pre-finalizer was used to unregister the Scrollbar from the theme.
The only implementation of the UnregisterScrollbar method that did
anything was that of ScrollbarThemeMac, which removed the scrollbar
from a HashSet and a HashMap. Both data-structures actually held
weak members, so the gc can automatically remove the ScrollBar from
the data-structures and the pre-finalizer is not needed.

The method UnregisterScrollbar was only called from the pre-finalizer,
so I removed it as well.

Bug: 981414
Change-Id: Ief15a9df658a12568de9fabdabd2950419835b56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720774Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681731}

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

@@ -72,10 +72,6 @@ class LayoutScrollbarTheme final : public ScrollbarTheme {
     return ScrollbarTheme::DeprecatedStaticGetTheme().RegisterScrollbar(
         scrollbar);
   }
-  void UnregisterScrollbar(Scrollbar& scrollbar) override {
-    return ScrollbarTheme::DeprecatedStaticGetTheme().UnregisterScrollbar(
-        scrollbar);
-  }
 
   int MinimumThumbLength(const Scrollbar&) override;
 

third_party/blink/renderer/core/scroll/scroll_animator_mac.mm

@@ -229,14 +229,8 @@ static NSSize abs(NSSize size) {
     scrollbar = _scrollableArea->VerticalScrollbar();
 
   // It is possible to have a null scrollbar here since it is possible for this
-  // delegate
-  // method to be called between the moment when a scrollbar has been set to 0
-  // and the
-  // moment when its destructor has been called. We should probably de-couple
-  // some
-  // of the clean-up work in ScrollbarThemeMac::unregisterScrollbar() to avoid
-  // this
-  // issue.
+  // delegate method to be called between the moment when a scrollbar has been
+  // set to 0 and the moment when its destructor has been called.
   if (!scrollbar)
     return NSZeroPoint;
 

third_party/blink/renderer/core/scroll/scrollbar.cc

@@ -85,10 +85,6 @@ Scrollbar::Scrollbar(ScrollableArea* scrollable_area,
 
 Scrollbar::~Scrollbar() =default;
 
-void Scrollbar::Dispose() {
-  theme_.UnregisterScrollbar(*this);
-}
-
 void Scrollbar::Trace(blink::Visitor* visitor) {
   visitor->Trace(scrollable_area_);
   visitor->Trace(chrome_client_);

third_party/blink/renderer/core/scroll/scrollbar.h

@@ -47,7 +47,6 @@ class WebMouseEvent;
 
 class CORE_EXPORT Scrollbar : public GarbageCollectedFinalized<Scrollbar>,
                               public DisplayItemClient {
-    USING_PRE_FINALIZER(Scrollbar, Dispose);
  public:
   // Theme object ownership remains with the caller and it must outlive the
   // scrollbar.
@@ -190,9 +189,6 @@ class CORE_EXPORT Scrollbar : public GarbageCollectedFinalized<Scrollbar>,
 
   CompositorElementId GetElementId();
 
-  // Promptly unregister from the theme manager + run finalizers of derived
-  // Scrollbars.
-  void Dispose();
   virtual void Trace(blink::Visitor*);
 
  protected:

third_party/blink/renderer/core/scroll/scrollbar_theme.h

@@ -190,7 +190,6 @@ class CORE_EXPORT ScrollbarTheme {
   }
 
   virtual void RegisterScrollbar(Scrollbar&) {}
-  virtual void UnregisterScrollbar(Scrollbar&) {}
 
   virtual bool IsMockTheme() const { return false; }
 

third_party/blink/renderer/core/scroll/scrollbar_theme_mac.h

@@ -43,7 +43,6 @@ class PLATFORM_EXPORT ScrollbarThemeMac : public ScrollbarTheme {
   ~ScrollbarThemeMac() override;
 
   void RegisterScrollbar(Scrollbar&) override;
-  void UnregisterScrollbar(Scrollbar&) override;
 
   bool SupportsControlTints() const override { return true; }
 

third_party/blink/renderer/core/scroll/scrollbar_theme_mac.mm

@@ -228,11 +228,6 @@ void ScrollbarThemeMac::RegisterScrollbar(Scrollbar& scrollbar) {
   UpdateScrollbarOverlayColorTheme(scrollbar);
 }
 
-void ScrollbarThemeMac::UnregisterScrollbar(Scrollbar& scrollbar) {
-  GetScrollbarPainterMap().erase(&scrollbar);
-  GetScrollbarSet().erase(&scrollbar);
-}
-
 void ScrollbarThemeMac::SetNewPainterForScrollbar(
     Scrollbar& scrollbar,
     ScrollbarPainter new_painter) {