Add virtual/rootlayerscrolls/scrollbars test suite.

third_party/WebKit/LayoutTests/VirtualTestSuites

 [
   {
     "prefix": "gpu",
     "base": "fast/canvas",
     "args": ["--enable-accelerated-2d-canvas",
              "--disable-display-list-2d-canvas"]
   },
   {
     "prefix": "display_list_2d_canvas",
     "base": "fast/canvas",
@@ skipping 102 matching lines @@
     "prefix": "rootlayerscrolls",
     "base": "fast/scrolling",
     "args": ["--root-layer-scrolls"]
   },
   {
     "prefix": "rootlayerscrolls",
     "base": "fast/history/scroll-restoration",
     "args": ["--root-layer-scrolls"]
   },
   {
+    "prefix": "rootlayerscrolls",
+    "base": "scrollbars",
+    "args": ["--root-layer-scrolls"]
+  },
+  {
     "prefix": "scroll_customization",
     "base": "fast/scroll-behavior",
     "args": ["--enable-blink-features=ScrollCustomization"]
   },
   {
     "prefix": "scroll_customization",
     "base": "fast/scrolling",
     "args": ["--enable-blink-features=ScrollCustomization"]
   },
   {
@@ skipping 41 matching lines @@
     "prefix": "threaded",
     "base": "inspector/tracing",
     "args": ["--enable-threaded-compositing"]
   },
   {
     "prefix": "spv2",
     "base": "paint",
     "args": ["--enable-slimming-paint-v2"]
   }
 ]