layout-tests: Add flag for including compositingReasons in layerTreeAsText().

LayoutTests/fast/dom/CompositorProxy/proxy-forces-layer.html

Index: LayoutTests/fast/dom/CompositorProxy/proxy-forces-layer.html
diff --git a/LayoutTests/fast/dom/CompositorProxy/proxy-forces-layer.html b/LayoutTests/fast/dom/CompositorProxy/proxy-forces-layer.html
index 4ecbf97fc7f542c0eeca61f1f566935de83c4aeb..254bf74871619cba0f852e3ce85771b7a93691e2 100644
--- a/LayoutTests/fast/dom/CompositorProxy/proxy-forces-layer.html
+++ b/LayoutTests/fast/dom/CompositorProxy/proxy-forces-layer.html
@@ -15,9 +15,10 @@ async_test(function(test) {
 
         // Creating a proxy should force the container to have a layer.
         var proxy = new CompositorProxy(container, ['opacity']);
-        docLayer = JSON.parse(window.internals.layerTreeAsText(document));
+        docLayer = JSON.parse(window.internals.layerTreeAsText(document, internals.LAYER_TREE_INCLUDES_COMPOSITING_REASONS));
         assert_equals(1, docLayer.children.length);
         assert_equals(1, docLayer.children[0].children.length);
+        assert_in_array('compositorProxy', docLayer.children[0].children[0].compositingReasons);
 
         // Detach the element from the document. The corresponding layer should be removed.
         container.parentNode.removeChild(container);
@@ -27,9 +28,10 @@ async_test(function(test) {
 
         // Add the element back to the document. The corresponding layer should reappear.
         document.body.appendChild(container);
-        docLayer = JSON.parse(window.internals.layerTreeAsText(document));
+        docLayer = JSON.parse(window.internals.layerTreeAsText(document, internals.LAYER_TREE_INCLUDES_COMPOSITING_REASONS));
         assert_equals(1, docLayer.children.length);
         assert_equals(1, docLayer.children[0].children.length);
+        assert_in_array('compositorProxy', docLayer.children[0].children[0].compositingReasons);
 
         // Disconnecting the proxy should also remove the layer.
         proxy.disconnect();