Disable tests that are flaky with Network Service

These two session cookies-related tests are flaky when the Network
Service is enabled.

Bug: 795417
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Idf1195bf51f15851ada6349426af8939a720056a
No-Presubmit: true
No-Try: true
Tbr: jam@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/830969Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524477}

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

@@ -824,3 +824,8 @@
 # Prevent tests from making real network requests.
 # https://crbug.com/793899
 -ExtensionApiTest.DoNotOpenUninstallUrlForBlacklistedExtensions
+
+# Session cookies tests are flaky.
+# https://crbug.com/795417
+-ContinueWhereILeftOffTest.SessionCookies
+-RestartTest.SessionCookies