Disable loading.desktop/TheOnion_warm on win 7

This is flaky on 'Win 7 Nvidia GPU Perf'

Bug:876636
Change-Id: Ide6e0ceff3b7963f86b7160345a3239542d476c1

TBR=kouhei@chromium.org, ksakamoto@chromium.org

No-Try: true
Change-Id: Ide6e0ceff3b7963f86b7160345a3239542d476c1
Reviewed-on: https://chromium-review.googlesource.com/1240316Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#593521}

tools/perf/expectations.config

@@ -65,6 +65,7 @@ crbug.com/867836 [ All ] loading.desktop/Elmundo_warm [ Skip ]
 crbug.com/876636 [ ChromeOS ] loading.desktop/TheOnion_cold [ Skip ]
 crbug.com/876636 [ ChromeOS ] loading.desktop/TheOnion_warm [ Skip ]
 crbug.com/876636 [ Win_10 ] loading.desktop/TheOnion_warm [ Skip ]
+crbug.com/876636 [ Win_7 ] loading.desktop/TheOnion_warm [ Skip ]
 crbug.com/876636 [ ChromeOS ] loading.desktop/AllRecipes_cold [ Skip ]
 crbug.com/876636 [ ChromeOS ] loading.desktop/AllRecipes_warm [ Skip ]
 crbug.com/879833 [ Mac_10.12 ] loading.desktop/AllRecipes_cold [ Skip ]