For SPv2, attach element to compositor animation player without a CLM.

third_party/WebKit/Source/core/animation/CompositorAnimations.cpp

Index: third_party/WebKit/Source/core/animation/CompositorAnimations.cpp
diff --git a/third_party/WebKit/Source/core/animation/CompositorAnimations.cpp b/third_party/WebKit/Source/core/animation/CompositorAnimations.cpp
index 6cb11d8f9b4916ff758cdec23ea0cad0e74525e9..e4524c769cdcaff03ad55521107a0244dc04a9cf 100644
--- a/third_party/WebKit/Source/core/animation/CompositorAnimations.cpp
+++ b/third_party/WebKit/Source/core/animation/CompositorAnimations.cpp
@@ -444,15 +444,18 @@ void CompositorAnimations::attachCompositedLayers(Element& element,
   if (!element.layoutObject() || !element.layoutObject()->isBoxModelObject())
     return;
 
-  PaintLayer* layer = toLayoutBoxModelObject(element.layoutObject())->layer();
+  // Composited animations do not depend on a composited layer mapping for SPv2.
+  if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
+    PaintLayer* layer = toLayoutBoxModelObject(element.layoutObject())->layer();
 
-  if (!layer || !layer->isAllowedToQueryCompositingState() ||
-      !layer->compositedLayerMapping() ||
-      !layer->compositedLayerMapping()->mainGraphicsLayer())
-    return;
+    if (!layer || !layer->isAllowedToQueryCompositingState() ||
+        !layer->compositedLayerMapping() ||
+        !layer->compositedLayerMapping()->mainGraphicsLayer())
+      return;
 
-  if (!layer->compositedLayerMapping()->mainGraphicsLayer()->platformLayer())
-    return;
+    if (!layer->compositedLayerMapping()->mainGraphicsLayer()->platformLayer())
+      return;
+  }
 
   CompositorAnimationPlayer* compositorPlayer = animation.compositorPlayer();
   compositorPlayer->attachElement(createCompositorElementId(

[wkorman, 2017/01/20 23:45:47]

This will end up at this check:

 
https://cs.chromium.org/chromium/src/cc/animation/animation_player.cc?q=anima...

  if (animation_host_)
      RegisterPlayer();

I'm not yet sure whether animation_host_ will be true when we need it to be.
Further changes may be needed but this seems an incremental step with what I
know currently.

[loyso (OOO), 2017/01/24 00:48:20]

cc::AnimationPlayer::animation_host_ is not nullptr if
1) Player is added to timeline
2) Timeline is added to AnimationHost.