Run CSSOM smooth scroll animations on the compositor when possible

LayoutTests/VirtualTestSuites

 [
   {
     "prefix": "gpu",
     "base": "fast/canvas",
     "args": ["--enable-accelerated-2d-canvas",
              "--disable-display-list-2d-canvas",
              "--enable-impl-side-painting"]
   },
   {
     "prefix": "gpu",
@@ skipping 50 matching lines @@
     "prefix": "threaded",
     "base": "animations",
     "args": ["--enable-threaded-compositing"]
   },
   {
     "prefix": "threaded",
     "base": "transitions",
     "args": ["--enable-threaded-compositing"]
   },
   {
+    "prefix": "threaded",
+    "base": "fast/scroll-behavior",
+    "args": ["--enable-threaded-compositing",
+             "--enable-prefer-compositing-to-lcd-text"]
+  },
+  {
     "prefix": "stable",
     "base": "webexposed",
     "args": ["--stable-release-mode"]
   },
   {
     "prefix": "stable",
     "base": "animations-unprefixed",
     "args": ["--stable-release-mode"]
   },
   {
@@ skipping 122 matching lines @@
     "prefix": "slimmingpaint",
     "base": "fast/multicol",
     "args": ["--enable-slimming-paint", "--enable-impl-side-painting", "--enable-region-based-columns"]
   },
   {
     "prefix": "rootlayerscrolls",
     "base": "fast/scrolling",
     "args": ["--root-layer-scrolls"]
   }
 ]