Include VisualViewport layers in layerTreeAsText output.

third_party/WebKit/LayoutTests/compositing/fixed-body-background-positioned-expected.txt

 {
   "layers": [
     {
+      "name": "Inner Viewport Container Layer",
+      "bounds": [800, 600]
+    },
+    {
+      "name": "Overscroll Elasticity Layer"
+    },
+    {
+      "name": "Page Scale Layer"
+    },
+    {
+      "name": "Inner Viewport Scroll Layer",
+      "bounds": [800, 600]
+    },
+    {
+      "name": "Frame Overflow Controls Host Layer",
+      "bounds": [800, 600]
+    },
+    {
       "name": "Frame Clipping Layer",
       "bounds": [785, 600]
     },
     {
       "name": "LayoutView #document (background) Layer",
       "transformOrigin": [0, 0],
       "bounds": [785, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "Frame Scrolling Layer"
     },
     {
       "name": "Content Root Layer",
       "bounds": [785, 3700]
     },
     {
       "name": "LayoutView #document",
       "bounds": [785, 3700],
       "drawsContent": true
+    },
+    {
+      "name": "Frame Vertical Scrollbar Layer",
+      "position": [785, 0],
+      "bounds": [15, 600],
+      "contentsOpaque": true
     }
   ]
 }