Disable PlatformAppBrowserTest.Restrictions in network_service_browser_tests

The test is flaky.

TBR=jam@chromium.org

Bug: 820995
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I2b0f7baa9491e29184595febf039fb2402358af5
Reviewed-on: https://chromium-review.googlesource.com/959014Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542515}

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

@@ -334,3 +334,7 @@
 
 # Flakes https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.linux%2FLinux_Tests%2F65817%2F%2B%2Frecipes%2Fsteps%2Fnetwork_service_browser_tests%2F0%2Flogs%2FRestartTest.CookiesClearedOnExit%2F0
 -RestartTest.CookiesClearedOnExit
+
+# Flaky in linux tests.
+# https://crbug.com/820995
+-PlatformAppBrowserTest.Restrictions