Attempt to fix the crash in blink Scroll Animator.

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 76 matching lines @@
     , m_scrollCornerNeedsPaintInvalidation(false)
 {
 }
 
 ScrollableArea::~ScrollableArea()
 {
 }
 
 void ScrollableArea::clearScrollAnimators()
 {
-#if OS(MACOSX) && ENABLE(OILPAN)

[ajuma, 2016/02/22 15:15:01]

Instead of using an #if, WDYT of adding a virtual method that has an empty
implementation in ScrollAnimatorCompositorCoordinator and whose implementation
in ScrollAnimatorMac calls dispose()?

[bokan, 2016/02/22 15:55:35]

Why do we need to manually call dispose here at all? The animator is GCd so
can't we just clear the pointer and let Oilpan call dispose when the time is
right?

If there's something else that we need to happen *right here* (e.g. I see
ScrollAnimatorMac invalidates some painting code) then we should be doing that
through a method other than dispose.

[ymalik, 2016/02/22 19:22:56]

I think they have this call here because if oilpan is not enabled, dispose would
be called by the destructor, but if oilpan is enabled, presumably we want to
"dispose" right when ~ScrollableArea is called and not wait for dispose to be
called by oilpan (this function is called by ~ScrollableArea).  

[bokan, 2016/02/22 19:35:46]

The only place I see this being called from is
PaintLayerScrollableArea::dispose(). With Oilpan on, destructors aren't called
at all IIRC. If we have something here that needs to happen at a specific
time/order, we shouldn't put it in the dispose method. Otherwise, just let
Oilpan handle it.

+#if ENABLE(OILPAN)
     if (m_scrollAnimator)
         m_scrollAnimator->dispose();
 #endif
     m_scrollAnimator.clear();
     m_programmaticScrollAnimator.clear();
 }
 
 ScrollAnimatorBase& ScrollableArea::scrollAnimator() const
 {
     if (!m_scrollAnimator)
@@ skipping 499 matching lines @@
         std::max(0, size.height() - horizontalScrollbarHeight));
 }
 
 DEFINE_TRACE(ScrollableArea)
 {
     visitor->trace(m_scrollAnimator);
     visitor->trace(m_programmaticScrollAnimator);
 }
 
 } // namespace blink