cc: Allow the main thread to cancel commits

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 879834d8649eae902ef6294ed9849b9885c02cb4..c1e45f6a41e42580c46ea4d9a9634384683806e3 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -388,7 +388,17 @@ class CC_EXPORT Layer : public base::RefCounted<Layer>,
 
   Layer();
 
+  // These SetNeeds functions are in order of severity of update:
+  //
+  // Called when this layer has been modified in some way, but isn't sure
+  // that it needs a commit yet.  It needs CalcDrawProperties and UpdateLayers
+  // before it knows whether or not a commit is required.
+  void SetNeedsUpdate();
+  // Called when a property has been modified in a way that the layer
+  // knows immediately that a commit is required.  This implies SetNeedsUpdate.
   void SetNeedsCommit();
+  // Called when there's been a change in layer structure.  Implies both
+  // SetNeedsUpdate and SetNeedsCommit.
   void SetNeedsFullTreeSync();
   bool IsPropertyChangeAllowed() const;