Adding janky transform test case

tools/perf/page_sets/tough_animation_cases.json

 {
   "description": "A collection of animation performance tests",
   "pages": [
     {
       "url": "file:///tough_animation_cases/keyframed_animations.html",
       "why": "Tests various keyframed animations.",
       "smoothness": {
         "action": "wait_interaction",
         "duration": 5
       }
     },
     {
       "url": "file:///tough_animation_cases/transform_transitions.html",
       "why": "Tests various transitions.",
       "smoothness": {
         "action": "wait_interaction",
         "duration": 5
       }
     },
     {
       "url": "http://ie.microsoft.com/testdrive/performance/robohornetpro/",
       "why": "Login page is slow because of ineffecient transform operations",
       "smoothness": {
         "action": "wait_interaction",
         "duration": 5
       }
+    },
+    {
+      "url": "file:///tough_animation_cases/transform_transition_js_block.html",
+      "why": "JS execution blocks CSS transition unless initial transform is set",
+      "smoothness": {
+        "action": "wait_interaction",
+        "duration": 5
+      }
     }
   ]
 }