Blink Compositor Animation: Introduce CompositorAnimationHost correctly.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
@@ skipping 1795 matching lines @@
 
 Widget* PaintLayerScrollableArea::getWidget() {
   return box().frame()->view();
 }
 
 void PaintLayerScrollableArea::resetRebuildScrollbarLayerFlags() {
   m_rebuildHorizontalScrollbarLayer = false;
   m_rebuildVerticalScrollbarLayer = false;
 }
 
+CompositorAnimationHost* PaintLayerScrollableArea::compositorAnimationHost()
+    const {
+  if (LocalFrame* frame = box().frame()) {

[jbroman, 2016/12/13 16:38:32]

OK because it already exists below, but ordinarily I'd prefer an early-return
style to progressively deeper nesting of if statements and ternary operators.

LocalFrame* frame = box().frame();
if (!frame)
  return nullptr;

Page* page = frame->page();
if (!page)
  return nullptr;

ScrollingCoordinator* coordinator = page->scrollingCoordinator();
if (!coordinator)
  return nullptr;

return coordinator->compositorAnimationHost();

[loyso (OOO), 2016/12/19 04:03:12]

Done.

+    if (Page* page = frame->page()) {
+      return page->scrollingCoordinator()
+                 ? page->scrollingCoordinator()->compositorAnimationHost()
+                 : nullptr;
+    }
+  }
+  return nullptr;
+}
+
 CompositorAnimationTimeline*
 PaintLayerScrollableArea::compositorAnimationTimeline() const {
   if (LocalFrame* frame = box().frame()) {
     if (Page* page = frame->page())
       return page->scrollingCoordinator()
                  ? page->scrollingCoordinator()->compositorAnimationTimeline()
                  : nullptr;
   }
   return nullptr;
 }
@@ skipping 207 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink