Disable flaky tests in network_service_browser_tests

Disable DeclarativeContentApiTest.{Dis,En}abledForSp{anning,split}Incognito for
network_service_browser_tests due to flakiness

TBR=jam@chromium.org

Bug: 798969
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I0f58b76ba43f2e17b2002ab0d767332f363a8832
Reviewed-on: https://chromium-review.googlesource.com/849834
Commit-Queue: Dominic Battré <battre@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526947}

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

@@ -881,3 +881,9 @@
 
 # Flakes https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.linux%2FLinux_Tests%2F65816%2F%2B%2Frecipes%2Fsteps%2Fnetwork_service_browser_tests%2F0%2Flogs%2FDeclarativeContentApiTest.IsBookmarkedRulesEvaluatedOnBookmarkEvents%2F0
 -DeclarativeContentApiTest.IsBookmarkedRulesEvaluatedOnBookmarkEvents
+
+# Flakes, crbug.com/798969
+-DeclarativeContentApiTest.DisabledForSpanningIncognito
+-DeclarativeContentApiTest.DisabledForSplitIncognito
+-DeclarativeContentApiTest.EnabledForSpanningIncognito
+-DeclarativeContentApiTest.EnabledForSpanningIncognito