Don't squash into RenderPart subclasses. Don't squash into video elements.

Source/core/rendering/compositing/CompositingLayerAssigner.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 103 matching lines @@
         }
     }
     return update;
 }
 
 CompositingReasons CompositingLayerAssigner::getReasonsPreventingSquashing(const RenderLayer* layer, const CompositingLayerAssigner::SquashingState& squashingState)
 {
     if (!squashingState.haveAssignedBackingsToEntireSquashingLayerSubtree)
         return CompositingReasonSquashingWouldBreakPaintOrder;
 
+    ASSERT(squashingState.hasMostRecentMapping);
+    const RenderLayer& squashingLayer = squashingState.mostRecentMapping->owningLayer();
+
     // FIXME: this special case for video exists only to deal with corner cases
     // where a RenderVideo does not report that it needs to be directly composited.
     // Video does not currently support sharing a backing, but this could be
     // generalized in the future. The following layout tests fail if we permit the
     // video to share a backing with other layers.
     //
     // compositing/video/video-controls-layer-creation.html
-    if (layer->renderer()->isVideo())
+    if (layer->renderer()->isVideo() || squashingLayer.renderer()->isVideo())
         return CompositingReasonSquashingVideoIsDisallowed;
 
     // Don't squash iframes, frames or plugins.
     // FIXME: this is only necessary because there is frame code that assumes that composited frames are not squashed.
-    if (layer->renderer()->isRenderPart())
+    if (layer->renderer()->isRenderPart() || squashingLayer.renderer()->isRenderPart())
         return CompositingReasonSquashingRenderPartIsDisallowed;
 
     if (layer->reflectionInfo())
         return CompositingReasonSquashingReflectionIsDisallowed;
 
     if (squashingWouldExceedSparsityTolerance(layer, squashingState))
         return CompositingReasonSquashingSparsityExceeded;
 
-    // FIXME: this is not efficient, since it walks up the tree . We should store these values on the CompositingInputsCache.
-    ASSERT(squashingState.hasMostRecentMapping);
-    const RenderLayer& squashingLayer = squashingState.mostRecentMapping->owningLayer();
-
+    // FIXME: this is not efficient, since it walks up the tree. We should store these values on the CompositingInputsCache.
     if (layer->clippingContainer() != squashingLayer.clippingContainer() && !squashingLayer.compositedLayerMapping()->containingSquashedLayer(layer->clippingContainer()))
         return CompositingReasonSquashingClippingContainerMismatch;
 
     // Composited descendants need to be clipped by a child containment graphics layer, which would not be available if the layer is
     // squashed (and therefore has no CLM nor a child containment graphics layer).
     if (m_compositor->clipsCompositingDescendants(layer))
         return CompositingReasonSquashedLayerClipsCompositingDescendants;
 
     if (layer->scrollsWithRespectTo(&squashingLayer))
         return CompositingReasonScrollsWithRespectToSquashingLayer;
@@ skipping 128 matching lines @@
 
     RenderLayerStackingNodeIterator iterator(*layer->stackingNode(), NormalFlowChildren | PositiveZOrderChildren);
     while (RenderLayerStackingNode* curNode = iterator.next())
         assignLayersToBackingsInternal(curNode->layer(), squashingState, layersNeedingPaintInvalidation);
 
     if (squashingState.hasMostRecentMapping && &squashingState.mostRecentMapping->owningLayer() == layer)
         squashingState.haveAssignedBackingsToEntireSquashingLayerSubtree = true;
 }
 
 }