Changes subtreeShouldRenderToSeparateSurface logic to account for explicit clipping

cc/layer_tree_host_common.cc

Index: cc/layer_tree_host_common.cc
diff --git a/cc/layer_tree_host_common.cc b/cc/layer_tree_host_common.cc
index 0a204ceb6914df96a5bdede2b6c8c9ee7e1c6915..90bc1c9228c41ad3e75be70db7a57d9f73b0bc42 100644
--- a/cc/layer_tree_host_common.cc
+++ b/cc/layer_tree_host_common.cc
@@ -274,7 +274,7 @@ static bool subtreeShouldRenderToSeparateSurface(LayerType* layer, bool axisAlig
         return true;
 
     // If the layer clips its descendants but it is not axis-aligned with respect to its parent.
-    if (layerClipsSubtree(layer) && !axisAlignedWithRespectToParent && numDescendantsThatDrawContent > 0)
+    if (layerClipsSubtree(layer) && !axisAlignedWithRespectToParent && !layer->drawProperties().can_clip_self)

[danakj, 2012/12/15 18:49:12]

Why doesn't a layer need to know that all layers in its entire subtree can each
clip themselves to their respective parents in order to decide it doesn't need
create and render the subtree into a render surface? Don't you need to check
subtreeCanEachClipToParent?

         return true;
 
     // If the layer has some translucency and does not have a preserves-3d transform style.
@@ -435,15 +435,25 @@ template<typename LayerType>
 static void preCalculateMetaInformation(LayerType* layer)
 {
     int numDescendantsThatDrawContent = 0;
+    bool canClipSelf = true;

[danakj, 2012/12/15 18:49:12]

Why isn't this initialized to the return for the current Layer::canClipSelf()
function?

Does that function actually mean allowClippingToParent?

+    bool sublayerXformPreventsClip = !layer->sublayerTransform().IsPositiveScaleOrTranslation();
 
     for (size_t i = 0; i < layer->children().size(); ++i) {
         LayerType* childLayer = layer->children()[i];
         preCalculateMetaInformation<LayerType>(childLayer);
+
         numDescendantsThatDrawContent += childLayer->drawsContent() ? 1 : 0;
         numDescendantsThatDrawContent += childLayer->drawProperties().num_descendants_that_draw_content;
+
+        if ((childLayer->drawsContent() && !childLayer->canClipSelf()) ||
+            !childLayer->drawProperties().can_clip_self ||
+            sublayerXformPreventsClip ||
+            !childLayer->transform().IsPositiveScaleOrTranslation())
+            canClipSelf = false;
     }
 
     layer->drawProperties().num_descendants_that_draw_content = numDescendantsThatDrawContent;
+    layer->drawProperties().can_clip_self = canClipSelf;
 }
 
 // Recursively walks the layer tree starting at the given node and computes all the