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

tools/perf/page_sets/tough_scrolling_cases.json

 {
   "description": "A collection of difficult scrolling tests",
   "smoothness": { "action": "scroll" },
   "pages": [
-    { "url": "file:///tough_scrolling_cases/background_fixed.html" },
-    { "url": "file:///tough_scrolling_cases/cust_scrollbar.html" },
-    { "url": "file:///tough_scrolling_cases/div_scrolls.html" },
-    { "url": "file:///tough_scrolling_cases/fixed_nonstacking.html" },
-    { "url": "file:///tough_scrolling_cases/fixed_stacking.html" },
-    { "url": "file:///tough_scrolling_cases/iframe_scrolls.html" },
-    { "url": "file:///tough_scrolling_cases/simple.html" },
-    { "url": "file:///tough_scrolling_cases/wheel_body_prevdefault.html" },
-    { "url": "file:///tough_scrolling_cases/wheel_div_prevdefault.html" }
+    { "url": "file://tough_scrolling_cases/background_fixed.html" },
+    { "url": "file://tough_scrolling_cases/cust_scrollbar.html" },
+    { "url": "file://tough_scrolling_cases/div_scrolls.html" },
+    { "url": "file://tough_scrolling_cases/fixed_nonstacking.html" },
+    { "url": "file://tough_scrolling_cases/fixed_stacking.html" },
+    { "url": "file://tough_scrolling_cases/iframe_scrolls.html" },
+    { "url": "file://tough_scrolling_cases/simple.html" },
+    { "url": "file://tough_scrolling_cases/wheel_body_prevdefault.html" },
+    { "url": "file://tough_scrolling_cases/wheel_div_prevdefault.html" }
   ]
 }