Mark nested-reflection-transition.html as flaky on Trusty

BUG=674468
TBR=schenney@chromium.org

Review-Url: https://codereview.chromium.org/2575383003 .
Cr-Commit-Position: refs/heads/master@{#438802}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2218,3 +2218,5 @@ crbug.com/674396 [ Win ] compositing/reflections/nested-reflection-transition.ht
 
 # Added 2016-12-15
 crbug.com/674466 [ Win10 ] fast/forms/select/menulist-appearance-rtl.html [ Pass Failure ]
+crbug.com/674468 [ Trusty ] compositing/reflections/nested-reflection-transition.html [ Pass Failure ]
+