Rename remaining x_stress_test pagesets tests to tough_x_cases convention

This rounds out my patch series to point image decoding benchmark at
image_decoding_benchmark.json but also get a tough_image_cases pageset
established.

NOTRY=True

Review URL: https://chromiumcodereview.appspot.com/11783086

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176139 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/page_sets/image_stress_tests.json

-{
-  "description": "A collection of image-heavy sites.",
-  "user_agent_type": "desktop",
-  "pages": [
-    {
-      "url": "http://www.free-pictures-photos.com/aviation/airplane-306.jpg"
-    },
-    {
-      "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"
-    }
-  ]
-}

tools/perf/page_sets/tough_image_cases.json

 {
-  "description": "**THIS IS OBSOLETE, EDIT image_decoding_benchmark.json instead***",
+  "description": "A collection of image-heavy sites.",
+  "user_agent_type": "desktop",
   "pages": [
     {
-      "url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?gif",
-      "post_navigate_javascript_to_execute": "runBenchmark();",
-      "wait_for_javascript_expression": "isDone",
-      "image_decoding_benchmark_limit_results_to_min_iterations": true
+      "url": "http://www.free-pictures-photos.com/aviation/airplane-306.jpg"
     },
     {
-      "url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?jpg",
-      "post_navigate_javascript_to_execute": "runBenchmark();",
-      "wait_for_javascript_expression": "isDone",
-      "image_decoding_benchmark_limit_results_to_min_iterations": true
-    },
-    {
-      "url": "file:///../../../chrome/test/data/image_decoding/image_decoding.html?png",
-      "post_navigate_javascript_to_execute": "runBenchmark();",
-      "wait_for_javascript_expression": "isDone",
-      "image_decoding_benchmark_limit_results_to_min_iterations": true
+      "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"
     }
   ]
 }