service worker: Disable a nav preload test under NetworkService.

ServiceWorkerNavigationPreloadTest.NetworkFallback is flaky when
NetworkService is on, after r585201. Temporarily disabling
to soothe the CQ while I investigate.

Bug: 876911,  873575
Change-Id: I8b5cdd9ae8ac173a2a6debba05219bad7fa0d606
TBR: kinuko
NOTRY: true
Reviewed-on: https://chromium-review.googlesource.com/1186003Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585343}

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

@@ -37,6 +37,10 @@
 # https://crbug.com/827318
 -RenderThreadImplBrowserTest.NonResourceDispatchIPCTasksDontGoThroughScheduler
 
+# Flaky, temporarily disabling while investigating.
+# https://crbug.com/876911
+-ServiceWorkerNavigationPreloadTest.NetworkFallback
+
 # NOTE: if adding an exclusion for an existing failure (e.g. additional test for
 # feature X that is already not working), please add it beside the existing
 # failures. Otherwise please reach out to network-service-dev@.