Rewrite blink->cc scroll offset logic for SPV2

third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp

Index: third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp b/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
index 3a58fa717bfd01cd5d48348c23141404cd32f360..e8a9c887ad6cada6a5e84c99276039991c7062a9 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
@@ -272,26 +272,21 @@ int PropertyTreeManager::ensureCompositorScrollNode(
   DCHECK(result.isNewEntry);
   scrollTree().set_needs_update(true);
 
-  return id;
-}
-
-void PropertyTreeManager::updateScrollOffset(int layerId, int scrollId) {
-  cc::ScrollNode& scrollNode = *scrollTree().Node(scrollId);
-  cc::TransformNode& transformNode =
-      *transformTree().Node(scrollNode.transform_id);
-
-  transformNode.scrolls = true;
-
   // Blink creates a 2d transform node just for scroll offset whereas cc's
   // transform node has a special scroll offset field. To handle this we
-  // adjust cc's transform node to remove the 2d scroll translation and
-  // let the cc scroll tree update the cc scroll offset.
-  DCHECK(transformNode.local.IsIdentityOr2DTranslation());
-  auto offset = transformNode.local.To2dTranslation();
-  transformNode.local.MakeIdentity();
-  scrollTree().SetScrollOffset(layerId,

[ajuma, 2017/01/20 21:35:48]

With this removed, is the scroll offset getting set on the scroll tree at all?
(If not, I'd expect main-thread-driven scrolling to work, but compositor-driven
scrolling to be broken.)

-                               gfx::ScrollOffset(-offset.x(), -offset.y()));
-  scrollTree().set_needs_update(true);
+  // adjust the compositor transform node to remove the 2d scroll translation
+  // and instead set a scroll offset.
+  cc::TransformNode& compositorTransformNode =
+      *transformTree().Node(compositorNode.transform_id);
+  DCHECK(compositorTransformNode.local.IsIdentityOr2DTranslation());
+  compositorTransformNode.local.MakeIdentity();
+  auto* scrollOffsetTransform = scrollNode->scrollOffsetTranslation();
+  auto scrollOffset = scrollOffsetTransform->matrix().to2DTranslation();
+  compositorTransformNode.scroll_offset =
+      gfx::ScrollOffset(-scrollOffset.width(), -scrollOffset.height());
+  compositorTransformNode.scrolls = true;
+
+  return id;
 }
 
 int PropertyTreeManager::switchToEffectNode(