Disable SocketApiTest.SocketUDPExtension with the NetworkService enabled

It flakily reaches a point of execution where extensions try to
do a host resolution using the RequestContext, which isn't set up when
the network service is enabled.

TBR=jam@chromium.org

Bug: 854762
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I5883a5620b953ca40fd582d39b96ea29f4f988e9
Reviewed-on: https://chromium-review.googlesource.com/1108737Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569027}

testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

@@ -116,6 +116,10 @@
 -PPAPINaClPNaClNonSfiTest.TCPServerSocketPrivate
 -PPAPINaClPNaClNonSfiTest.TCPSocket
 -PPAPINaClPNaClNonSfiTest.TCPSocketPrivate
+# This test fails flakily. Only enable it once
+# SocketExtensionWithDnsLookupFunction::StartDnsLookup uses the Mojo
+# HostResolver API.
+-SocketApiTest.SocketUDPExtension
 
 # Uncategorized PPAPI timeouts or test failures.
 -OutOfProcessPPAPITest.HostResolver