Change LayerImpl::LayerTreeAsJson to return a std::unique_ptr.

cc/layers/ui_resource_layer_impl.cc

Index: cc/layers/ui_resource_layer_impl.cc
diff --git a/cc/layers/ui_resource_layer_impl.cc b/cc/layers/ui_resource_layer_impl.cc
index 5e3768df8066a423d9e53eae3dcadfa171e6abf5..1b67d111f4a2daaa09576371a22c287f8264bb08 100644
--- a/cc/layers/ui_resource_layer_impl.cc
+++ b/cc/layers/ui_resource_layer_impl.cc
@@ -140,8 +140,9 @@ const char* UIResourceLayerImpl::LayerTypeAsString() const {
   return "cc::UIResourceLayerImpl";
 }
 
-base::DictionaryValue* UIResourceLayerImpl::LayerTreeAsJson() const {
-  base::DictionaryValue* result = LayerImpl::LayerTreeAsJson();
+std::unique_ptr<base::DictionaryValue> UIResourceLayerImpl::LayerTreeAsJson()
+    const {
+  std::unique_ptr<base::DictionaryValue> result = LayerImpl::LayerTreeAsJson();
 
   result->Set("ImageBounds", MathUtil::AsValue(image_bounds_).release());