Mark close-code-and-reason.html flaky in virtual

Mark
virtual/off-main-thread-websocket/http/tests/websocket/close-code-and-reason.html
as flaky.

BUG=831320

Change-Id: I388617a9d8269da334bfa3d1de1cdafbb8203efb
Reviewed-on: https://chromium-review.googlesource.com/1009782
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550176}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2285,6 +2285,7 @@ crbug.com/803200 virtual/off-main-thread-websocket/external/wpt/websockets/openi
 crbug.com/825740 virtual/off-main-thread-websocket/http/tests/websocket/multiple-connections-throttled.html [ Timeout ]
 crbug.com/825740 virtual/off-main-thread-websocket/http/tests/websocket/workers/close-code-and-reason.html [ Failure ]
 crbug.com/825740 virtual/off-main-thread-websocket/http/tests/websocket/workers/close.html [ Failure ]
+crbug.com/831320 virtual/off-main-thread-websocket/http/tests/websocket/close-code-and-reason.html [ Pass Failure ]
 
 crbug.com/805463 external/wpt/acid/acid3/numbered-tests.html [ Skip ]