Mark a layout test flaky

The specific one is
virtual/blink-cors/http/tests/security/contentSecurityPolicy/require-sri-for/require-sri-for-script-preload-allowed.php

The original test is already marked flaky.

TBR=gayane@chromium.org
NOTRY=true

Bug: 678499
Change-Id: I7d220e6aa12bd4079d392021e387e7cac1422fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1644935Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666251}

third_party/blink/web_tests/TestExpectations

@@ -4343,6 +4343,7 @@ crbug.com/678346 [ Win7 Debug ] storage/indexeddb/index-cursor.html [ Pass Timeo
 
 crbug.com/678493 http/tests/permissions/chromium/test-request-window.html [ Timeout Pass ]
 crbug.com/678499 http/tests/security/contentSecurityPolicy/require-sri-for/require-sri-for-script-preload-allowed.php [ Failure Pass ]
+crbug.com/678499 virtual/blink-cors/http/tests/security/contentSecurityPolicy/require-sri-for/require-sri-for-script-preload-allowed.php [ Failure Pass ]
 
 crbug.com/747751 [ Win ] http/tests/devtools/application-panel/resources-panel-resource-preview.js [ Failure Pass ]