cc: Do not cleanup tiles with raster tasks.

cc/layers/picture_layer_impl.h

Index: cc/layers/picture_layer_impl.h
diff --git a/cc/layers/picture_layer_impl.h b/cc/layers/picture_layer_impl.h
index 3b07c5dfd367897ce1e7b7524846c66c1908072a..0a687efc4bc6e605686eea9e89bc6d763f7abcd3 100644
--- a/cc/layers/picture_layer_impl.h
+++ b/cc/layers/picture_layer_impl.h
@@ -112,6 +112,7 @@ class CC_EXPORT PictureLayerImpl
   virtual size_t GetMaxTilesForInterestArea() const OVERRIDE;
   virtual float GetSkewportTargetTimeInSeconds() const OVERRIDE;
   virtual int GetSkewportExtrapolationLimitInContentPixels() const OVERRIDE;
+  virtual WhichTree GetTree() const OVERRIDE;
 
   // PushPropertiesTo active tree => pending tree.
   void SyncTiling(const PictureLayerTiling* tiling);
@@ -126,7 +127,6 @@ class CC_EXPORT PictureLayerImpl
 
   // Functions used by tile manager.
   PictureLayerImpl* GetTwinLayer() { return twin_layer_; }
-  WhichTree GetTree() const;
   bool IsOnActiveOrPendingTree() const;
   bool HasValidTilePriorities() const;
   bool AllTilesRequiredForActivationAreReadyToDraw() const;