[telemetry] Support absolute paths by changing file:/// to file://. More robust file path handling.

tools/perf/page_sets/tough_animation_cases.json

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