Disable broken page in key_silk_cases.

tools/perf/page_sets/key_silk_cases.json

 {
   "description": "Pages hand-picked for project Silk.",
   "archive_data_file": "data/key_silk_cases.json",
   "credentials_path": "data/credentials.json",
   "user_agent_type": "mobile",
   "smoothness": { "action": "scroll" },
   "navigate_steps" : [
     { "action": "navigate" },
     { "action": "wait", "seconds": 2 }
   ],
@@ skipping 313 matching lines @@
       "navigate_steps" : [
         { "action": "navigate" },
         { "action": "wait", "seconds": 1 }
       ],
       "smoothness": {
         "action": "scroll",
         "scrollable_element_function": "function(callback) { callback(document.getElementsByClassName('container')[0]); }"
       }
     },
     {
+      "disabled": "Times out on Windows; crbug.com/338838",
       "url": "http://plus.google.com/app/basic/stream",
       "credentials": "google",
       "navigate_steps" : [
         { "action": "navigate" },
         { "action": "wait", "javascript": "document.getElementsByClassName('fHa').length > 0" },
         { "action": "wait", "seconds": 2 }
       ],
       "smoothness": {
         "action": "scroll",
         "scrollable_element_function": "function(callback) { callback(document.getElementById('mainContent')); }"
@@ skipping 127 matching lines @@
         "wait_after": { "seconds": 3 }
       },
       "close_signup_box": {
         "action": "tap",
         "element_function": "function(callback) { callback(document.getElementsByClassName('signup-hide')[0]); }",
         "wait_after": { "javascript": "document.getElementsByClassName('signup-hide')[0].getBoundingClientRect().left < 0" }
       }
     }
   ]
 }