Mark http/tests/serviceworker/foreign-fetch-cors.html flaky on Win

Failed on the following builds, amongst others:
7052 7040 7035 7031 7024 7023 6993 6988 6974
Bot: https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win7%20%28dbg%29?numbuilds=200

BUG=640947
TBR=mek@chromium.org
NOTRY=true
NOTREECHECKS=true

Review-Url: https://codereview.chromium.org/2278133002
Cr-Commit-Position: refs/heads/master@{#414418}

third_party/WebKit/LayoutTests/TestExpectations

@@ -786,6 +786,8 @@ crbug.com/480769 http/tests/inspector/service-workers/service-worker-agents.html
 
 crbug.com/569901 [ Debug ] http/tests/serviceworker/navigation-redirect.html [ Pass Failure ]
 
+crbug.com/640947 [ Win ] http/tests/serviceworker/foreign-fetch-cors.html [ Pass Failure ]
+
 crbug.com/599670 [ Win Linux ] http/tests/inspector/resource-parameters-ipv6.html [ Failure Pass ]
 
 crbug.com/472330 fast/borders/border-image-outset-split-inline-vertical-lr.html [ Failure ]