Label feature-policy-frame-policy-allowed-for-some.https.sub.html timeout

The test has been timing out on win7_chromium_rel_ng and
linux_chromium_rel_ng.

TBR=loonybear@chromium.org

Bug: 829921
Change-Id: I87950fa86cccfe00a0d5b02778c551216bc32e02
Reviewed-on: https://chromium-review.googlesource.com/999912
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548869}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4029,3 +4029,6 @@ crbug.com/826936 external/wpt/webauthn/getcredential-timeout.https.html [ Pass T
 
 # Flaky on all platforms
 crbug.com/829761 external/wpt/cookie-store/cookieStore_special_names.tentative.https.html [ Pass Failure ]
+
+# Sheriff 2018-04-06
+crbug.com/829921 external/wpt/feature-policy/feature-policy-frame-policy-allowed-for-some.https.sub.html [ Pass Timeout ]