Update test filter for --enable-network-service

The test times out randomly (~10%) since Build #13171:
http/tests/security/powerfulFeatureRestrictions/old-powerful-features-on-insecure-origin.html

TBR=jam@chromium.org

Bug: 729849
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I2eec60a97836aff470894faaaebc2e3260cce996
Reviewed-on: https://chromium-review.googlesource.com/1062073
Commit-Queue: Chong Zhang <chongz@chromium.org>
Reviewed-by: Chong Zhang <chongz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559169}

third_party/WebKit/LayoutTests/FlagExpectations/enable-features=NetworkService

@@ -37,6 +37,7 @@ Bug(none) http/tests/security/cors-rfc1918 [ Crash Timeout ]
 Bug(none) http/tests/security/mime-type-execute-as-html-01.html [ Failure ]
 Bug(none) http/tests/security/mime-type-execute-as-html-04.html [ Failure ]
 Bug(none) http/tests/security/mixedContent/filesystem-url-in-iframe.html [ Failure Timeout ]
+Bug(none) http/tests/security/powerfulFeatureRestrictions/old-powerful-features-on-insecure-origin.html [ Pass Timeout ]
 
 # Cross-Origin Read Blocking is not implemented.
 crbug.com/792546 http/tests/inspector-protocol/network/block_cross_site_document_load.js [ Failure ]