Blink Compositor Animation: Introduce CompositorAnimationHost correctly.

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

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index a6adbe381105a5087e2a60c4f45c7312390da0fa..2d04c1d25133c72b0d08532c0b9c26db2c4b5d96 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1813,6 +1813,18 @@ void PaintLayerScrollableArea::resetRebuildScrollbarLayerFlags() {
   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()) {