Disable broken page in key_silk_cases.

The following page always errors out inside rasterize_and_record.key_silk_cases
on some windows platforms:
  http://plus.google.com/app/basic/stream

This change disables this page so that the perf bots can pass this test for the
other pages.

BUG=338838
R=dominikg@chromium.org, tonyg@chromium.org

Review URL: https://codereview.chromium.org/167143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251358 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/page_sets/key_silk_cases.json

@@ -331,6 +331,7 @@
       }
     },
     {
+      "disabled": "Times out on Windows; crbug.com/338838",
       "url": "http://plus.google.com/app/basic/stream",
       "credentials": "google",
       "navigate_steps" : [