[telemetry] Clean up image decoding benchmark

tools/perf/page_sets/tough_image_cases.json

 {
   "description": "A collection of image decoding performance tests",
   "pages": [
-    {"url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?gif"},
-    {"url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?jpg"},
-    {"url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?png"}
+    {
+      "url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?gif",
+      "post_navigate_javascript_to_execute": "runBenchmark();",
+      "wait_for_javascript_expression": "isDone",
+      "is_image_decoding_benchmark": true
+    },
+    {
+      "url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?jpg",
+      "post_navigate_javascript_to_execute": "runBenchmark();",
+      "wait_for_javascript_expression": "isDone",
+      "is_image_decoding_benchmark": true
+    },
+    {
+      "url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?png",
+      "post_navigate_javascript_to_execute": "runBenchmark();",
+      "wait_for_javascript_expression": "isDone",
+      "is_image_decoding_benchmark": true
+    },
+    {
+      "url": "http://www.free-pictures-photos.com/aviation/airplane-306.jpg"

[tonyg, 2013/01/04 23:26:32]

This test is currently running on the bots, so we need a .wpr recording. Could
you please check that in before you check this in? Hopefully everything will
just work even though there is a combination of file: and http: URLs.

+    },
+    {
+      "url": "http://upload.wikimedia.org/wikipedia/commons/c/cb/General_history%2C_Alaska_Yukon_Pacific_Exposition%2C_fully_illustrated_-_meet_me_in_Seattle_1909_-_Page_78.jpg"
+    }
   ]
 }